Queen's gambit declined Slav : why 5.a4?

Hello everyone!

I was studying the queen's gambit the other day and I am wondering why the move 5. a4 is best in this opening :  1. d4 d5 2. c4 dxc4 3. Nf3 Nf6 4. Nc3 c6

Here's the position :

It is the usual continuation and my chess engine also says so. Why is it better than, say, 5. e3?

Thanks.

Because, though a4 does fight for control of b5, it doesn't do the same as e3, which is prepare for recapture and simultaneously prepare to fight in the center.

Black's threat after 4... dxc4 (usually on move 4, not move 2, in the mainline Slav) is to play ...b5 and hold the pawn.  After 5. e3 b5, white has a harder time regaining the pawn.  5. a4 controls the b5 square, so if black tries 5... b5, white regains the pawn immediately with 6. axb5 cxb5 7. Nxb5.

But still, why not just push a4 at a later point. I feel it would be stronger just to develop and gain a position edge for any pawns your opponent wastes time trying to hold on to.

Drizztd this is why a4 is important.....

I see. Thanks for the correction. That makes a lot of sense.

good info, thanks

timeless_thoughts' diagram is what I was referring to in my post.  After 5. e3 b5, white has a harder time regaing the pawn (and developing effectively) than after 5. a4.

Yes, my diagram is perfect nor our blacks moves, but I hope you guys get the point

5 a4 is the main line in the 4...dxc4 slav but both  5 e4 and  5 e3 are playable and a lot of fun as well ! Kasparov himself has played 5 e4 !?  before  ( Geller gambit )

Would have been more fun it it had been Kasparov through a poltergeist.

 Seems like Karpov also likes e3 a lot.  But most of us are not him, which is why we play a4.