user2307: In the following opening:
[FEN ""]
1. d4 d5 2. c4 dxc4 3. e3 b5 4. a4

There is a well-known trap if Black tries to defend the c file pawn with:
[FEN ""]
1. d4 d5 2. c4 dxc4 3. e3 b5 4. a4 c6

However I got the following played against me:
[FEN ""]
1. d4 d5 2. c4 dxc4 3. e3 b5 4. a4 Bd7

How should White respond?

user2307: After giving the issue some thought I think White has better prospects with:
[fen ""]
1. d4 d5 2. c4 dxc4 3. e3 b5 4. a4 Bd7 5. axb5 Bxb5 6. Nc3 *

At the very least a tempo is won. If Black is not very careful as to where he retreats his Bishop he may very well lose material in addition to the c4 pawn which is doomed by all accounts.

user2042: [fen ""]

1.d4 d5 2.c4 dxc4 3.e3 b5? 4. a4! Bd7? 5.axb5! Bxb5 6.Bxc4! Bxc4 7.Qxa4+ 

and White gets back the bishop with the advantage ( no matter what Black plays Qxc4 is White's next move ).
Hopefully this answer helped.
Best regards.

2307: <p>In the following opening:</p>

<pre><code>[FEN ""]
1. d4 d5 2. c4 dxc4 3. e3 b5 4. a4
</code></pre>

<p>There is a well-known trap if Black tries to defend the c file pawn with:</p>

<pre><code>[FEN ""]
1. d4 d5 2. c4 dxc4 3. e3 b5 4. a4 c6
</code></pre>

<p>However I got the following played against me:</p>

<pre><code>[FEN ""]
1. d4 d5 2. c4 dxc4 3. e3 b5 4. a4 Bd7
</code></pre>

<p>How should White respond?</p>
 2307: <p>After giving the issue some thought I think White has better prospects with:</p>

<pre><code>[fen ""]
1. d4 d5 2. c4 dxc4 3. e3 b5 4. a4 Bd7 5. axb5 Bxb5 6. Nc3 *
</code></pre>

<p>At the very least a tempo is won. If Black is not very careful as to where he retreats his Bishop he may very well lose material in addition to the c4 pawn which is doomed by all accounts.</p>
 2042: <pre><code>[fen ""]

1.d4 d5 2.c4 dxc4 3.e3 b5? 4. a4! Bd7? 5.axb5! Bxb5 6.Bxc4! Bxc4 7.Qxa4+ 
</code></pre>

<p>and White gets back the bishop with the advantage ( no matter what Black plays <code>Qxc4</code> is White's next move ).</p>

<p>Hopefully this answer helped.</p>

<p>Best regards.</p>
