user3757: In a game with a friend, he made a really unexpected move, e3:
[FEN "rnbqkbnr/pppppppp/8/8/8/4P3/PPPP1PPP/RNBQKBNR w KQkq - 0 1"]

Afterwards, when analysing the game I found its name is Van't Kruijs Opening and I was wondering which are the ideas to play it.
For white, this seems a passive move, giving advantage in the centre to black. But on the other side, like e4, it opens the way to the King's Bishop and Queen. It can also lead to aggressive variations like the Keoni-Hiva Gambit, where white sacrifices a pawn for development, which seems dangerous.
[FEN ""]
1. e3 d5 2. Nc3 e5 3. f4

Could you confirm/refute this analysis? Could you discuss other ideas?

user12716: I play 3 minutes games on a smartphone and am usually around 1500-1600. In my personal experience it works well for me for the following reasons:
I open with the second bishop also (sometimes called ultramodern opening) and against weaker players I sometimes get the knights pawn on the opposing end and then a rook too because sometimes people just don't see a long diagonal.
If the opponent opens with a knight on the third row I can pin that knight to the king and get that pawn in the center. 
In some worst case scenarios I double the opponents pawns and develop a little better.
One of the main ideas of these openings is giving up the center at first and then gaining it a bit later.
Also, I feel my game is stronger in the mid and tactical part when I still have knights so exchanging bishops early makes this possible and also makes it easier for me to think about castling sides and combinations if my opponent misses the black or white bishop...

user11629: In my view a chess opening is characterized by a typical pawn structure and plans associated with it. As such a single move like 1. e3 should not deserve to be called an "opening' because there are no ideas behind it besides that it allows you to develop queen and bishop. True it can transpose to somewhat decent openings like the Colle system, reversed Stonewall, reversed French and the English, but it is impossible to give you a single "idea" that is behind all these openings. Similarly you would not get a good answer asking for the "ideas" in 1.e4 openings, because it can turn into so many different openings each with its own ideas. Call it "too broad" if you prefer.
OK, 1. e3 is not the worst first move you can make, but it is unambitious, basically giving black free choice in the final pawn structure in the center. 
In my opinion the only "ideas" behind 1. e3 are not found on the board, but rather in  

getting the opponent out of book, and
surprise. 

It is nothing that black should fear and does not require any preparation for black either, as you can just play whatever setup you want against it.
As for the Keoni-Hiva Gambit and similar suspicious lines, I don't see how playing f4 helps white's development. Perhaps if black gets greedy and starts eating pawns (fxe3, exd2) white can get a lead in development but black is not forced to do so and just playing normally I'd be happy to face the Keoni-Hiva Gambit with black.

user3757: I post this answer for completeness and in response to @Dag Oskar Madsen's comment. According to the ICCF games database, after 1. e3 d5 2. Nc3 e5 3. Qh5!? the main line is the following one:
[FEN ""]
1.e3 d5 2.Nc3 e5 3.Qh5 Nc6 4.Bb5 Qd6 5.d4 exd4 6.exd4 Nf6 ( 6...a6 )


user12699: I'm surprised no one has yet mentioned, none other than Magnus Carlsen recently played 1. e3 in the Olympiad. His teammates said that when he told them his intentions, they suddenly realized what a "punchable face" he had, but he ended up winning a very nice game. As for the opening, it transposed into a well-known variation of the Queen's Indian Defense.
If you see a strong player open with 1. e3, their intentions are probably like Magnus's: to transpose to another opening, perhaps after confusing their opponent a bit.
Take the variations given on Wikipedia and the like with a grain of salt. For example, the so-called Keoni-Hiva Gambit, 1.e3 d5 2.Nc3 Nf6 3.a3 e5 4.f4 exf4 5.Nf3, is nothing more than a series of mistakes by white, leaving black a pawn up with a better position. My computer already gives -1.3.

user-1: I've come across 1. e3 from one of my regular opponents quite often. These are my observations for why White might like this line:

Psychological: White plays a non-theoretical, flexible opening that White is likely to have played far more than Black. 
Familiarity with an opening uses clock time more efficiently. Black may have to spend more time on the opening moves to check that there are no tactical deficiencies
As you rightly state, the move liberates the Queen & Bishop
Transposition tricks: White has the opportunity to transpose to the English, reversed French, reversed Dutch & Queen pawn openings, e.g. the Colle
Solidity: White has a solid structure which facilitates safe development & early castling

The Van't Krujis leads to many opening lines, the named ones from Wikipedia are:
[title "Keoni-Hiva Gambit"]    
[fen ""]

1.e3 d5 2.Nc3 Nf6 3.a3 e5 4.f4 exf4 5.Nf3


White gambits the pawn to castle kingside & target f7
White often plays b5 to either fianchetto the queen side bishop to attack the king side
Black sometimes jettisons the d5 pawn to the c3 knight with Nxd5 to reduce White's initiative
Also, 3. a3 prevents a disruptive Bb4, threatening to remove the knight & simplifying when Black will soon be up a pawn

and
[title "Amsterdam Attack"]    
[fen ""]

1.e3 e5 2.c4 d6 3.Nc3 Nc6 4.b3 Nf6


This is a transposition to the English opening, Bremen line

and
[title "Ekolu Variation"]    
[fen ""]

1.e3 e5 2.Nc3 d5 3.f4 exf4 4.Nf3


Similar analysis to the Keoni-Hiva Gambit

and
[title "Alua Variation"]    
[fen ""]

1.e3 e5 2.Nc3 Nc6 3.f4 exf4 4.Nf3


Similar analysis to the Keoni-Hiva Gambit

and
[title "Akahi Variation"]    
[fen ""]

1.e3 e5 2.Nc3 Nf6 3.f4 exf4 4.Nf3


Similar analysis to the Keoni-Hiva Gambit

and
[title "Bouncing Bishop Variation"]    
[fen ""]

 1.e3 e5 2.Bc4 d5 3.Bb3


White aims to pressure the centre & Black's kingside with the Bishop on b3, similar to lines of the Ruy Lopez where White plays Bf1-b5-a4-b3


user2258: White can transpose to the Stonewall or the Colle systems after 1...d5 or if 1...e5 attempt a reverse French.
Otherwise 1. e3 is a move which needs no attention or search for ideas, because there can be none.

3757: <p>In a game with a friend, he made a really unexpected move, e3:</p>

<pre><code>[FEN "rnbqkbnr/pppppppp/8/8/8/4P3/PPPP1PPP/RNBQKBNR w KQkq - 0 1"]
</code></pre>

<p>Afterwards, when analysing the game I found its name is Van't Kruijs Opening and I was wondering which are the ideas to play it.</p>

<p>For white, this seems a passive move, giving advantage in the centre to black. But on the other side, like e4, it opens the way to the King's Bishop and Queen. It can also lead to aggressive variations like the Keoni-Hiva Gambit, where white sacrifices a pawn for development, which seems dangerous.</p>

<pre><code>[FEN ""]
1. e3 d5 2. Nc3 e5 3. f4
</code></pre>

<p>Could you confirm/refute this analysis? Could you discuss other ideas?</p>
 12716: <p>I play 3 minutes games on a smartphone and am usually around 1500-1600. In my personal experience it works well for me for the following reasons:</p>

<p>I open with the second bishop also (sometimes called ultramodern opening) and against weaker players I sometimes get the knights pawn on the opposing end and then a rook too because sometimes people just don't see a long diagonal.</p>

<p>If the opponent opens with a knight on the third row I can pin that knight to the king and get that pawn in the center. </p>

<p>In some worst case scenarios I double the opponents pawns and develop a little better.</p>

<p>One of the main ideas of these openings is giving up the center at first and then gaining it a bit later.</p>

<p>Also, I feel my game is stronger in the mid and tactical part when I still have knights so exchanging bishops early makes this possible and also makes it easier for me to think about castling sides and combinations if my opponent misses the black or white bishop...</p>
 11629: <p>In my view a chess opening is characterized by a typical pawn structure and plans associated with it. As such a single move like 1. e3 should not deserve to be called an "opening' because there are no ideas behind it besides that it allows you to develop queen and bishop. True it can transpose to somewhat decent openings like the Colle system, reversed Stonewall, reversed French and the English, but it is impossible to give you a single "idea" that is behind all these openings. Similarly you would not get a good answer asking for the "ideas" in 1.e4 openings, because it can turn into so many different openings each with its own ideas. Call it "too broad" if you prefer.</p>

<p>OK, 1. e3 is not the worst first move you can make, but it is unambitious, basically giving black free choice in the final pawn structure in the center. </p>

<p>In my opinion the only "ideas" behind 1. e3 are not found on the board, but rather in  </p>

<ol>
<li>getting the opponent out of book, and</li>
<li>surprise. </li>
</ol>

<p>It is nothing that black should fear and does not require any preparation for black either, as you can just play whatever setup you want against it.</p>

<p>As for the Keoni-Hiva Gambit and similar suspicious lines, I don't see how playing f4 helps white's development. Perhaps if black gets greedy and starts eating pawns (fxe3, exd2) white can get a lead in development but black is not forced to do so and just playing normally I'd be happy to face the Keoni-Hiva Gambit with black.</p>
 3757: <p>I post this answer for completeness and in response to @Dag Oskar Madsen's comment. According to the ICCF games database, after 1. e3 d5 2. Nc3 e5 3. Qh5!? the main line is the following one:</p>

<pre><code>[FEN ""]
1.e3 d5 2.Nc3 e5 3.Qh5 Nc6 4.Bb5 Qd6 5.d4 exd4 6.exd4 Nf6 ( 6...a6 )
</code></pre>
 12699: <p>I'm surprised no one has yet mentioned, <a href="http://www.chessgames.com/perl/chessgame?gid=1830402" rel="nofollow noreferrer">none other than Magnus Carlsen recently played 1. e3 in the Olympiad</a>. His teammates said that when he told them his intentions, they suddenly realized what a "punchable face" he had, but he ended up winning a very nice game. As for the opening, it transposed into a well-known variation of the Queen's Indian Defense.</p>

<p>If you see a strong player open with 1. e3, their intentions are probably like Magnus's: to transpose to another opening, perhaps after confusing their opponent a bit.</p>

<p>Take the variations given on Wikipedia and the like with a grain of salt. For example, the so-called Keoni-Hiva Gambit, 1.e3 d5 2.Nc3 Nf6 3.a3 e5 4.f4 exf4 5.Nf3, is nothing more than a series of mistakes by white, leaving black a pawn up with a better position. My computer already gives -1.3.</p>
 -1: <p>I've come across 1. e3 from one of my regular opponents quite often. These are my observations for why White might like this line:</p>

<ol>
<li>Psychological: White plays a non-theoretical, flexible opening that White is likely to have played far more than Black. </li>
<li>Familiarity with an opening uses clock time more efficiently. Black may have to spend more time on the opening moves to check that there are no tactical deficiencies</li>
<li>As you rightly state, the move liberates the Queen &amp; Bishop</li>
<li>Transposition tricks: White has the opportunity to transpose to the English, reversed French, reversed Dutch &amp; Queen pawn openings, e.g. the Colle</li>
<li>Solidity: White has a solid structure which facilitates safe development &amp; early castling</li>
</ol>

<p>The Van't Krujis leads to many opening lines, the named ones from Wikipedia are:</p>

<pre><code>[title "Keoni-Hiva Gambit"]    
[fen ""]

1.e3 d5 2.Nc3 Nf6 3.a3 e5 4.f4 exf4 5.Nf3
</code></pre>

<ul>
<li>White gambits the pawn to castle kingside &amp; target f7</li>
<li>White often plays b5 to either fianchetto the queen side bishop to attack the king side</li>
<li>Black sometimes jettisons the d5 pawn to the c3 knight with Nxd5 to reduce White's initiative</li>
<li>Also, 3. a3 prevents a disruptive Bb4, threatening to remove the knight &amp; simplifying when Black will soon be up a pawn</li>
</ul>

<p>and</p>

<pre><code>[title "Amsterdam Attack"]    
[fen ""]

1.e3 e5 2.c4 d6 3.Nc3 Nc6 4.b3 Nf6
</code></pre>

<ul>
<li>This is a transposition to the English opening, Bremen line</li>
</ul>

<p>and</p>

<pre><code>[title "Ekolu Variation"]    
[fen ""]

1.e3 e5 2.Nc3 d5 3.f4 exf4 4.Nf3
</code></pre>

<ul>
<li>Similar analysis to the Keoni-Hiva Gambit</li>
</ul>

<p>and</p>

<pre><code>[title "Alua Variation"]    
[fen ""]

1.e3 e5 2.Nc3 Nc6 3.f4 exf4 4.Nf3
</code></pre>

<ul>
<li>Similar analysis to the Keoni-Hiva Gambit</li>
</ul>

<p>and</p>

<pre><code>[title "Akahi Variation"]    
[fen ""]

1.e3 e5 2.Nc3 Nf6 3.f4 exf4 4.Nf3
</code></pre>

<ul>
<li>Similar analysis to the Keoni-Hiva Gambit</li>
</ul>

<p>and</p>

<pre><code>[title "Bouncing Bishop Variation"]    
[fen ""]

 1.e3 e5 2.Bc4 d5 3.Bb3
</code></pre>

<ul>
<li>White aims to pressure the centre &amp; Black's kingside with the Bishop on b3, similar to lines of the Ruy Lopez where White plays Bf1-b5-a4-b3</li>
</ul>
 2258: <p>White can transpose to the Stonewall or the Colle systems after 1...d5 or if 1...e5 attempt a reverse French.</p>

<p>Otherwise 1. e3 is a move which needs no attention or search for ideas, because there can be none.</p>
