user11071: The move e2e4 could be played on move 1. (One half-move)
The move e7xd6 could be played no earlier than by black on move 3 (6 half-moves).
What move(s) has the latest "earliest possible move it could be played"?
A move is distinguished by:

what type of piece is being moved
the square the piece moves from
the square the piece moves to
whether it is a capture
what piece it is being promoted to, if any (probably not actually pertinent, but still)

(And notably, not by whether it causes check or checkmate, nor which colour the piece is, nor which piece [if any] is being captured, nor if it's en passant.)

user10380: ... Ba1xPb2 entails a black pawn getting to a2, then a1=B. This needs 12 plies. For example:

Nc3 b5
Rb1 b4
~ b3
~ bxa2
~ a1=B
~ Bxb2


user30158: Bbg3
This is what I came up with after seeing the other answers and wondering if promotion could make this take longer and while I haven't explored it too much, I think that this bishop under-promotion idea is definitely one to consider.
[FEN ""]
1. d4 c5 2. dxc5 Nh6 3. c6 Ng8 4. c7 Nh6 5. cxb8=B Ng8 6. Bcf4 Nh6 7. h4 Ng8 8. Bh2 Nh6 9. Bbg3


user32606: Great question!
Here's another try: bxa1=B+
Edit: Boo! Didn't read the passus about + being irrelevant!
So here's a cheat that needs 16 plies.
[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"]

1. b4 c5 2. c3 cxb4 3. Qa4 Qb6 4. Na3 bxa3 5. Bb2 Nf6 6. Kd1 h6 7. Kc2 axb2 8. Kb1 bxa1=B+


user11071: I thought I had an answer which was 18 half-moves, but actually only 11 half-moves.
Ra7xh7. Assuming it's black's rook, I think this takes 7 turns to move all the pawns out of the way, one to move the rook, then one more for the capture. Meanwhile, white has to get a pawn up to h7. (Making it a capture prevents white from helping by clearing out one of the other pawns). So, 18 half-moves.
However, if it's white's rook:  black moves pawns on c, d, e, f and g. Meanwhile, white plays a4, ra3, rb3, rxb7, rxa7. 11 half-moves.
[fen ""]

1. a4 c6 2. Ra3 d6 3. Rb3 e6 4. Rxb7 f6 5. Rxa7 g6 6. Rxh7


user10380: Qh8-a8.
As a White move this can happen on White's 9th i.e. the 17th ply:
1 e3 h5 2 Qxh5 Nf6 3 Qxh8 d6 4 Qxf8+ Kd7 5 Qxd8+ Kc6 6 Qxc8 Na6 7 Qxa8 g6 8 Qh8 g5 9 Qa8.
It seems not to matter much whether the queen goes from a to h or h to a.

user19278: I thought Rb2-h2 could be better than Rb1 etc, but JeopardyTempest found a 12 ply variant: 1. d4 h6 2. Bxh6 Rxh6 3. e4 Rc6 4. f4 Rxc2 5. g4 Rxb2 6. h4 Rh2
(and scuttled other ideas I had)
So, it seems 13 is the fastest possible.
Some analysis: Pawns reach any place in 5 moves (10 ply) pretty easily. Knight takes 4 (8 ply). Bishop seems to need at most 12 ply (Bg1-a7, Be1-Ba5). King does K(x)a8 in 12 ply (white uses knight to remove rook, black moves b7 and d7 pawns and king 4 times). Castling is 5th move (9 ply). So, the only remaining pieces are queen and rook, which have been attempted in many other answers with the maximum of 13 ply. Out of unmentioned possibilities, Qa2-h2 is doable in 12 and I can't think of any other rook or queen move that is a contender for 13+.

user30389: What about Kh1xh2?
15 plies is achievable:
[FEN ""]
1. h4 d6 2. f3 Qd7 3. Nh3 Qxh3 4. Kf2 Qxh1 5. h5 Qh4+ 6. Kg1 Qg3 7. Kh1 Qh2+ 8. Kxh2

Similarly I also only found a 15 half move solution for Ka1xa2:
[FEN ""]
1. d3 Nc6 2. Bh6 Nb4 3. Kd2 Nxc2 4. Kc1 Nxa1 5. Nd2 Nc2 6. Kb1 Nb4 7. Ka1 Nxa2 8. Kxa2

Let's consider the latter and see if we can do better. If we could, white would only have 7 moves.
The obvious one first: It needs to be the white king. The black king would need 8 moves just to do Ka1a2 on an otherwise empty board.
If we ignore castling for now, the white king has to move 4 times to reach a1 and once more to capture on a2.That only leaves 2 of white's moves to spare. On its way to a1 at least four pieces - the a1 rook and the piece or pawn on the b, c, and d file - either have to move out of the way or have to be captured. Since we established white has only 2 non-king moves this means black has to capture at least two of them and additionally land on a2, all within 6 moves. Can black actually use 2 pieces for that? I don't think so. If black used 2 different pieces, one has to survive and make it to a2. But the fastest black can land any piece on a2 is in 3 moves, namely with the h2 knight. In the remaining 3 moves black can only do one capture tho, since the distance is too large for knights and for queen/bishop one move has to be spent to open up the piece, one move to line up the diagonal (since neither bishop or the queen target anything on the a-d files), which leaves only one capture which we have determined is not enough to help white.
So we determined that black can only infiltrate with one piece.
Further, notice that if white were to clear a1 by themself they need to be careful not to block the path of their King again, i.e., not leave the rook on the back rank which they've spent a move to clear. But also moving the rook along the a-file takes two moves since the a-pawn would have to move first. There is the possibility that black clears the a-pawn, then moves out of the way, white plays Ra2 and black captures on e2 but this simply takes too much time since black needs 3 moves at least to capture on a2 in the first place.
This means the one black piece doing the infiltration must capture the rook and end on a2 at the same time. The only piece that can do this is the queen.
Notice that the first capture (that clears the way for the king) can only occur on black's third move. Also white may not capture the queen afterwards since we already determined one piece has to do all the infiltration and land on a2 at the end. This essentially still blocks the square the queen captured on and even worse, it also blocks all adjacent squares. Lastly, black's queen must be on a2 after black's 6th move but somehow has to allow the white king to reach a1 on white's 6th move. It's quite clear that this implies that black's 5th move can not be in white's queenside since otherwise the queen would block the king's access to a1.
So to summarize (without casling):

White has to move the king on all but 2 moves
Black must land the queen on a2 on their 6th move
Black must retreat the queen on their 5th move to allow the king to reach a1
Black can not capture on their first 3 moves

The first and last statement imply that within white's first 3 moves they have to move their king and since black cannot capture they have to make way themself. This leaves only a d-pawn move into Kd2. As soon as the king is on d2 the a3 bishop may no longer be captured since the king would be forced to take black's queen (or run from his target square). This means white has to move the bishop out, essentially locking down the first 3 moves: d3 (or d4) - bh6 (or g5/f4/e3) - kd2. From here white would have to move the king on every move and remember that at this point black still would not have had the chance to capture. It's not too hard to see that  this move sequence will not work.

So this leaves castling as our only option. But that does not work fast enough either. After castling, the king needs 3 to make more moves (including Ka1xa2) which leaves 3 moves to clear the back rank. However, clearing the back rank certainly requires 4 moves: d4 - qd3 - bd2 - nc3. Black can also not be of any help since even if they captured on the back rank (which they couldn't even do fast enough) and moved out, they'd cover the square on which they took, disallowing white to castle.

So long story short: I'm pretty sure Ka1xa2 is only possible as the 15th half move and not earlier.

user10695: Some more candidates that couldn't break the 13 mark, but may've equaled it, until someone with sharper eyes comes along:
13 Re1-e8: 1. Nf3 e5 2. d4 e4 3. Kd2 exf3 4. exf3 Ke7 5. Bc4 Kf6 6. Re1 Kf5 7. Re8
13 Re2xe8: 1. f4 e5 2. g3 Ke7 3. h4 exf4 4. Rh2 f3 5. exf3 Kd6 6. Re2 Qe8 7. Rxe8
13 Rd2-d8: 1. Nc3 e5 2. d4 exd4 3. Bf4 dxc3 4. Qxd7+ Qxd7 5. Rd1 Qg4 6. Rd2 Qxg2 7. Rd8+
11 Re2-e8: 1. f4 e5 2. g3 Qe7 3. h4 exf4 4. Rh2 Qxe2+ 5. Rxe2+ Kd8 6. Re8+

user14079: Ra1-h1
This is a similar idea to Rosie's, but using a rook instead of queen makes it longer for the far away side to achieve this move.
I believe it takes 15 plies (edit: actually 13 plies, see comments) for White to play this move :
1.Na3 e6 2.e4 Qf6 3.Qh5 Qxb2 4.Ba6 Qxb1 5.Ke2 Qxc1 6.h4 Qxg1 7.Rh3 Qh2 8.Ra1-h1
[FEN ""]
1.Na3 e6 2.e4 Qf6 3.Qh5 Qxb2 4.Ba6 Qxb1 5.Ke2 Qxc1 6.h4 Qxg1 7.Rh3 Qh2 8.Ra1h1

As a side note, on the other side of the board, Ra8-h8 seems to take only 14 plies :
1.e3 Nh6 2.Qf3 e5 3.Qxb7 Ba3 4.Qxb8 Qh4 5.Qxc8 Ke7 6.Qxh8 a6 7.Qxh7 Ra8-h8
[FEN ""]
1.e3 Nh6 2.Qf3 e5 3.Qxb7 Ba3 4.Qxb8 Qh4 5.Qxc8 Ke7 6.Qxh8 a6 7.Qxh7 Ra8h8


user14488: Rosie F proposed the move Qh8-a8. I can reach this move in 13 plies (White's 7th move):
[fen ""]

1. c4 e5 2. Qa4 Qh4 3. Qxa7 Ba3 4. Qxb8 Nf6 5. Qxc8+ Ke7 6. Qxh8 Ra4 7. Qa8

The key idea to is ensure that Black's 6th move is useful, which is accomplished by having the white queen open the way from the black queen's rook to escape.

user30173: Ka8-b8 (14 plies)
Earliest I could find requires 14 half-moves.
[FEN ""]
1. c3 d6 2. Qb3 Bh3 3. Qxb7 Kd7 4. Qxa8 Kc8 5. Qxb8+ Kxb8 6. c4 Ka8 7. c5 Kb8


Other king moves from a8 seem to work out the same.
Ka8xa7 (14 plies)
[FEN ""]
1. Nc3 d6 2. Nb5 Kd7 3. Nxc7 Kc6 4. Nxa8 b6 5. Nc7 Kb7 6. Nb5 Ka8 7. Nxa7 Kxa7

I couldn't prove that less moves is impossible. However, it requires starting with the pawn at d7, then reaching a8 by moving the Black king 4 times. Seems difficult even with help from White pieces.

user21223: Evargalo's Ra1-h1 for 15 plies is not correct, since this can be done in 13:
[FEN ""]
1. e3 h5 2. Qxh5 Rxh5 3. Bc4 Rxh2 4. Nf3 Rxh1+ 5. Ke2 Rxc1 6. Nc3 Rxc2 7. Rh1

However, it cannot be done in fewer than 13, making it a reasonable candidate. We can prove this as follows.
Suppose White can do it in 11 plies. This means seven White pieces get off the first rank (either by moving or being captured) in the first 10 plies. Since White only moves on five of those plies, at least two pieces get captured, and that means another ply is required to get the capturing piece off the first rank (or capture it). That is eight different plies accounted for.
Now Black cannot capture on the first rank with either of their first two moves. So all five of White's plies and the remaining three of Black's are needed to get pieces off the first rank. In particular, White makes no pawn moves. But if White makes no pawn moves, Black can't capture on the first rank with any of their first three moves, and now there aren't enough moves left.
Next, suppose Black can achieve it in 12 plies. Now we have one extra White piece to get off the first rank, but the capturing Black piece doesn't have to move off the first rank (as long as it is a rook). So in order to get everything clear in time, we need White to make at most one move which is not taking a piece off the first rank, but Black captures on the back rank with a rook on ply 6. This isn't possible - there's certainly not enough time to capture anywhere other than a1 or h1, and there's no way to get rid of both h pawns in the time (note that capturing one of them doesn't help, since the capturing piece still has to get off the file, and e.g. 1 g4 h5 2 h3 hg 3 hg Rxh1 isn't valid because White has made more than one pawn move).
[edit] In response to Sandro's comment, Rh1-a1 can also be done in 13 plies (and this is optimal for the same reasons).
[FEN ""]
1. b3 e6 2. e3 Qf6 3. Qf3 Qxa1 4. Bd3 Qxb1 5. Nh3 Qxc1+ 6. Ke2 Qb2 7. Ra1

[edit] As pointed out by corsiKa, Ra8-h8 and Rh8-a8 both require 14 plies. The analysis is similar to the above, except for Black to complete in 12 plies requires Black to move at least 4 pieces of the back rank first, with White capturing the other three and then moving off (this has to be done with the queen, since a bishop can't make two consecutive captures on the back rank, and the rooks can't get there in time). This means Black can't make more than one pawn move. That has to be the d, e or f pawn to allow the black king to get off the back rank in one move, and it has to be a light-squared pawn to allow the white queen to capture on the back rank on ply 5. However, if it is the f pawn White must eventually capture the f bishop (it can't move), but this forces the king to move at least twice since it can't stay on f7. So suppose Black moves the d pawn instead. Now White must play 3. Qxc8. If the black queen has already moved, this is check and the king can't move to d7. If not, the black queen is pinned and eventually White will play Qxd8, which forces a second king move. So this can't be done. (It's possible for Black to do either move in 14 plies - just swap sides in the examples above, with White playing any additional first move that doesn't get in the way.)
White can't do it in 13 plies. This would require either Black to make exactly one pawn move with White capturing with a rook on the back rank on ply 7, or two with White getting a rook to the back rank on ply 5, neither of which can be done.

11071: <p>The move e2e4 could be played on move 1. (One half-move)</p>
<p>The move e7xd6 could be played no earlier than by black on move 3 (6 half-moves).</p>
<p>What move(s) has the latest &quot;earliest possible move it could be played&quot;?</p>
<p>A move is distinguished by:</p>
<ul>
<li>what type of piece is being moved</li>
<li>the square the piece moves from</li>
<li>the square the piece moves to</li>
<li>whether it is a capture</li>
<li>what piece it is being promoted to, if any (probably not actually pertinent, but still)</li>
</ul>
<p>(And notably, not by whether it causes check or checkmate, nor which colour the piece is, nor which piece [if any] is being captured, nor if it's en passant.)</p>
 10380: <p>... Ba1xPb2 entails a black pawn getting to a2, then a1=B. This needs 12 plies. For example:</p>
<ol>
<li>Nc3 b5</li>
<li>Rb1 b4</li>
<li>~ b3</li>
<li>~ bxa2</li>
<li>~ a1=B</li>
<li>~ Bxb2</li>
</ol>
 30158: <p><strong>Bbg3</strong></p>
<p>This is what I came up with after seeing the other answers and wondering if promotion could make this take longer and while I haven't explored it too much, I think that this bishop under-promotion idea is definitely one to consider.</p>
<pre><code>[FEN &quot;&quot;]
1. d4 c5 2. dxc5 Nh6 3. c6 Ng8 4. c7 Nh6 5. cxb8=B Ng8 6. Bcf4 Nh6 7. h4 Ng8 8. Bh2 Nh6 9. Bbg3
</code></pre>
 32606: <p>Great question!</p>
<p>Here's another try: <strong>bxa1=B+</strong></p>
<p><em>Edit</em>: Boo! Didn't read the passus about + being irrelevant!</p>
<p>So here's a cheat that needs 16 plies.</p>
<pre><code>[FEN &quot;rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1&quot;]

1. b4 c5 2. c3 cxb4 3. Qa4 Qb6 4. Na3 bxa3 5. Bb2 Nf6 6. Kd1 h6 7. Kc2 axb2 8. Kb1 bxa1=B+
</code></pre>
 11071: <p>I thought I had an answer which was 18 half-moves, but actually only 11 half-moves.</p>
<p>Ra7xh7. Assuming it's black's rook, I think this takes 7 turns to move all the pawns out of the way, one to move the rook, then one more for the capture. Meanwhile, white has to get a pawn up to h7. (Making it a capture prevents white from helping by clearing out one of the other pawns). So, 18 half-moves.</p>
<p>However, if it's white's rook:  black moves pawns on c, d, e, f and g. Meanwhile, white plays a4, ra3, rb3, rxb7, rxa7. 11 half-moves.</p>
<pre><code>[fen &quot;&quot;]

1. a4 c6 2. Ra3 d6 3. Rb3 e6 4. Rxb7 f6 5. Rxa7 g6 6. Rxh7
</code></pre>
 10380: <p>Qh8-a8.</p>
<p>As a White move this can happen on White's 9th i.e. the 17th ply:</p>
<p>1 e3 h5 2 Qxh5 Nf6 3 Qxh8 d6 4 Qxf8+ Kd7 5 Qxd8+ Kc6 6 Qxc8 Na6 7 Qxa8 g6 8 Qh8 g5 9 Qa8.</p>
<p>It seems not to matter much whether the queen goes from a to h or h to a.</p>
 19278: <p>I thought Rb2-h2 could be better than Rb1 etc, but JeopardyTempest found a 12 ply variant: 1. d4 h6 2. Bxh6 Rxh6 3. e4 Rc6 4. f4 Rxc2 5. g4 Rxb2 6. h4 Rh2
(and scuttled other ideas I had)</p>
<p>So, it seems 13 is the fastest possible.</p>
<p>Some analysis: Pawns reach any place in 5 moves (10 ply) pretty easily. Knight takes 4 (8 ply). Bishop seems to need at most 12 ply (Bg1-a7, Be1-Ba5). King does K(x)a8 in 12 ply (white uses knight to remove rook, black moves b7 and d7 pawns and king 4 times). Castling is 5th move (9 ply). So, the only remaining pieces are queen and rook, which have been attempted in many other answers with the maximum of 13 ply. Out of unmentioned possibilities, Qa2-h2 is doable in 12 and I can't think of any other rook or queen move that is a contender for 13+.</p>
 30389: <p>What about <code>Kh1xh2</code>?</p>
<p>15 plies is achievable:</p>
<pre><code>[FEN &quot;&quot;]
1. h4 d6 2. f3 Qd7 3. Nh3 Qxh3 4. Kf2 Qxh1 5. h5 Qh4+ 6. Kg1 Qg3 7. Kh1 Qh2+ 8. Kxh2
</code></pre>
<p>Similarly I also only found a 15 half move solution for <code>Ka1xa2</code>:</p>
<pre><code>[FEN &quot;&quot;]
1. d3 Nc6 2. Bh6 Nb4 3. Kd2 Nxc2 4. Kc1 Nxa1 5. Nd2 Nc2 6. Kb1 Nb4 7. Ka1 Nxa2 8. Kxa2
</code></pre>
<p>Let's consider the latter and see if we can do better. If we could, white would only have 7 moves.</p>
<p>The obvious one first: It needs to be the white king. The black king would need 8 moves just to do <code>Ka1a2</code> on an otherwise empty board.</p>
<p>If we ignore castling for now, the white king has to move 4 times to reach a1 and once more to capture on a2.That only leaves 2 of white's moves to spare. On its way to a1 at least four pieces - the a1 rook and the piece or pawn on the b, c, and d file - either have to move out of the way or have to be captured. Since we established white has only 2 non-king moves this means black has to capture at least two of them and additionally land on a2, all within 6 moves. Can black actually use 2 pieces for that? I don't think so. If black used 2 different pieces, one has to survive and make it to a2. But the fastest black can land any piece on a2 is in 3 moves, namely with the h2 knight. In the remaining 3 moves black can only do one capture tho, since the distance is too large for knights and for queen/bishop one move has to be spent to open up the piece, one move to line up the diagonal (since neither bishop or the queen target anything on the a-d files), which leaves only one capture which we have determined is not enough to help white.</p>
<p>So we determined that black can only infiltrate with one piece.</p>
<p>Further, notice that if white were to clear a1 by themself they need to be careful not to block the path of their King again, i.e., not leave the rook on the back rank which they've spent a move to clear. But also moving the rook along the a-file takes two moves since the a-pawn would have to move first. There is the possibility that black clears the a-pawn, then moves out of the way, white plays Ra2 and black captures on e2 but this simply takes too much time since black needs 3 moves at least to capture on a2 in the first place.</p>
<p>This means the one black piece doing the infiltration must capture the rook and end on a2 at the same time. The only piece that can do this is the queen.</p>
<p>Notice that the first capture (that clears the way for the king) can only occur on black's third move. Also white may not capture the queen afterwards since we already determined one piece has to do all the infiltration and land on a2 at the end. This essentially still blocks the square the queen captured on and even worse, it also blocks all adjacent squares. Lastly, black's queen must be on a2 after black's 6th move but somehow has to allow the white king to reach a1 on white's 6th move. It's quite clear that this implies that black's 5th move can not be in white's queenside since otherwise the queen would block the king's access to a1.</p>
<p>So to summarize (without casling):</p>
<ul>
<li>White has to move the king on all but 2 moves</li>
<li>Black must land the queen on a2 on their 6th move</li>
<li>Black must retreat the queen on their 5th move to allow the king to reach a1</li>
<li>Black can not capture on their first 3 moves</li>
</ul>
<p>The first and last statement imply that within white's first 3 moves they have to move their king and since black cannot capture they have to make way themself. This leaves only a d-pawn move into Kd2. As soon as the king is on d2 the a3 bishop may no longer be captured since the king would be forced to take black's queen (or run from his target square). This means white has to move the bishop out, essentially locking down the first 3 moves: d3 (or d4) - bh6 (or g5/f4/e3) - kd2. From here white would have to move the king on every move and remember that at this point black still would not have had the chance to capture. It's not too hard to see that  this move sequence will not work.</p>
<hr />
<p>So this leaves castling as our only option. But that does not work fast enough either. After castling, the king needs 3 to make more moves (including Ka1xa2) which leaves 3 moves to clear the back rank. However, clearing the back rank certainly requires 4 moves: d4 - qd3 - bd2 - nc3. Black can also not be of any help since even if they captured on the back rank (which they couldn't even do fast enough) and moved out, they'd cover the square on which they took, disallowing white to castle.</p>
<hr />
<p>So long story short: I'm pretty sure <code>Ka1xa2</code> is only possible as the 15th half move and not earlier.</p>
 10695: <p>Some more candidates that couldn't break the 13 mark, but may've equaled it, until someone with sharper eyes comes along:</p>
<p>13 Re1-e8: 1. Nf3 e5 2. d4 e4 3. Kd2 exf3 4. exf3 Ke7 5. Bc4 Kf6 6. Re1 Kf5 7. Re8<br />
13 Re2xe8: 1. f4 e5 2. g3 Ke7 3. h4 exf4 4. Rh2 f3 5. exf3 Kd6 6. Re2 Qe8 7. Rxe8<br />
13 Rd2-d8: 1. Nc3 e5 2. d4 exd4 3. Bf4 dxc3 4. Qxd7+ Qxd7 5. Rd1 Qg4 6. Rd2 Qxg2 7. Rd8+<br />
11 Re2-e8: 1. f4 e5 2. g3 Qe7 3. h4 exf4 4. Rh2 Qxe2+ 5. Rxe2+ Kd8 6. Re8+</p>
 14079: <p><strong>Ra1-h1</strong></p>
<p>This is a similar idea to Rosie's, but using a rook instead of queen makes it longer for the far away side to achieve this move.</p>
<p>I believe it takes 15 plies (<em>edit</em>: actually <strong>13 plies</strong>, see comments) for White to play this move :</p>
<p>1.Na3 e6 2.e4 Qf6 3.Qh5 Qxb2 4.Ba6 Qxb1 5.Ke2 Qxc1 6.h4 Qxg1 7.Rh3 Qh2 8.Ra1-h1</p>
<pre><code>[FEN &quot;&quot;]
1.Na3 e6 2.e4 Qf6 3.Qh5 Qxb2 4.Ba6 Qxb1 5.Ke2 Qxc1 6.h4 Qxg1 7.Rh3 Qh2 8.Ra1h1
</code></pre>
<p>As a side note, on the other side of the board, Ra8-h8 seems to take only 14 plies :</p>
<p>1.e3 Nh6 2.Qf3 e5 3.Qxb7 Ba3 4.Qxb8 Qh4 5.Qxc8 Ke7 6.Qxh8 a6 7.Qxh7 Ra8-h8</p>
<pre><code>[FEN &quot;&quot;]
1.e3 Nh6 2.Qf3 e5 3.Qxb7 Ba3 4.Qxb8 Qh4 5.Qxc8 Ke7 6.Qxh8 a6 7.Qxh7 Ra8h8
</code></pre>
 14488: <p>Rosie F proposed the move Qh8-a8. I can reach this move in 13 plies (White's 7th move):</p>
<pre><code>[fen &quot;&quot;]

1. c4 e5 2. Qa4 Qh4 3. Qxa7 Ba3 4. Qxb8 Nf6 5. Qxc8+ Ke7 6. Qxh8 Ra4 7. Qa8
</code></pre>
<p>The key idea to is ensure that Black's 6th move is useful, which is accomplished by having the white queen open the way from the black queen's rook to escape.</p>
 30173: <h3>Ka8-b8 (14 plies)</h3>
<p>Earliest I could find requires 14 half-moves.</p>
<pre><code>[FEN &quot;&quot;]
1. c3 d6 2. Qb3 Bh3 3. Qxb7 Kd7 4. Qxa8 Kc8 5. Qxb8+ Kxb8 6. c4 Ka8 7. c5 Kb8
</code></pre>
<hr />
<p>Other king moves from a8 seem to work out the same.</p>
<h3>Ka8xa7 (14 plies)</h3>
<pre><code>[FEN &quot;&quot;]
1. Nc3 d6 2. Nb5 Kd7 3. Nxc7 Kc6 4. Nxa8 b6 5. Nc7 Kb7 6. Nb5 Ka8 7. Nxa7 Kxa7
</code></pre>
<p>I couldn't prove that less moves is impossible. However, it requires starting with the pawn at d7, then reaching a8 by moving the Black king 4 times. Seems difficult even with help from White pieces.</p>
 21223: <p>Evargalo's <code>Ra1-h1</code> for 15 plies is not correct, since this can be done in 13:</p>
<pre><code>[FEN &quot;&quot;]
1. e3 h5 2. Qxh5 Rxh5 3. Bc4 Rxh2 4. Nf3 Rxh1+ 5. Ke2 Rxc1 6. Nc3 Rxc2 7. Rh1
</code></pre>
<p>However, it cannot be done in fewer than 13, making it a reasonable candidate. We can prove this as follows.</p>
<p>Suppose White can do it in 11 plies. This means seven White pieces get off the first rank (either by moving or being captured) in the first 10 plies. Since White only moves on five of those plies, at least two pieces get captured, and that means another ply is required to get the capturing piece off the first rank (or capture it). That is eight different plies accounted for.</p>
<p>Now Black cannot capture on the first rank with either of their first two moves. So all five of White's plies and the remaining three of Black's are needed to get pieces off the first rank. In particular, White makes no pawn moves. But if White makes no pawn moves, Black can't capture on the first rank with any of their first three moves, and now there aren't enough moves left.</p>
<p>Next, suppose Black can achieve it in 12 plies. Now we have one extra White piece to get off the first rank, but the capturing Black piece doesn't have to move off the first rank (as long as it is a rook). So in order to get everything clear in time, we need White to make at most one move which is not taking a piece off the first rank, but Black captures on the back rank with a rook on ply 6. This isn't possible - there's certainly not enough time to capture anywhere other than a1 or h1, and there's no way to get rid of both h pawns in the time (note that capturing one of them doesn't help, since the capturing piece still has to get off the file, and e.g. 1 g4 h5 2 h3 hg 3 hg Rxh1 isn't valid because White has made more than one pawn move).</p>
<p>[edit] In response to Sandro's comment, Rh1-a1 can also be done in 13 plies (and this is optimal for the same reasons).</p>
<pre><code>[FEN &quot;&quot;]
1. b3 e6 2. e3 Qf6 3. Qf3 Qxa1 4. Bd3 Qxb1 5. Nh3 Qxc1+ 6. Ke2 Qb2 7. Ra1
</code></pre>
<p>[edit] As pointed out by corsiKa, Ra8-h8 and Rh8-a8 both require 14 plies. The analysis is similar to the above, except for Black to complete in 12 plies requires Black to move at least 4 pieces of the back rank first, with White capturing the other three and then moving off (this has to be done with the queen, since a bishop can't make two consecutive captures on the back rank, and the rooks can't get there in time). This means Black can't make more than one pawn move. That has to be the d, e or f pawn to allow the black king to get off the back rank in one move, and it has to be a light-squared pawn to allow the white queen to capture on the back rank on ply 5. However, if it is the f pawn White must eventually capture the f bishop (it can't move), but this forces the king to move at least twice since it can't stay on f7. So suppose Black moves the d pawn instead. Now White must play 3. Qxc8. If the black queen has already moved, this is check and the king can't move to d7. If not, the black queen is pinned and eventually White will play Qxd8, which forces a second king move. So this can't be done. (It's possible for Black to do either move in 14 plies - just swap sides in the examples above, with White playing any additional first move that doesn't get in the way.)</p>
<p>White can't do it in 13 plies. This would require either Black to make exactly one pawn move with White capturing with a rook on the back rank on ply 7, or two with White getting a rook to the back rank on ply 5, neither of which can be done.</p>
