411 REFERENCE · Wikibooks
Chess Opening Theory/1. d4/1...d5/2. c4/2...c6/3. Nc3/3...dxc4/4. e3
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Slav Defence D15
4. e3
This is considered to be the best move for White:
Keeps the e4-square free for the c3-knight to retreat to in case of a b7-b5-b4 advance.
Gains control of the centre with pawns on e3 and d4.
Prepares to capture the c4 pawn with Bxc4.
Black should prevent the capture of the c-pawn with /4...b5/. This usually leads to very complicated variations on the queenside where White attempts to undermine Black's defence of the c-pawn with moves like a4. If White can successfully recapture the pawn, White should be better.
Theory table
4. ... b5 5. a4 b4 6. Ne4 Qd5 7. Nd2 c3 8. bxc3 bxc3 9. Nb1 Qa5 + 4. ... ... 5. ... ... 6. ... ... 7. Ng3 Nf6 8. Be2 g6 9. Nf3 O-O 4. ... ... 5. Nxb5? cxb5 6. Qf3 --
See also
Slav Defence 3...dxc4