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Chess Opening Theory/1. e4/1...e6/2. d4/2...d5/3. Nc3/3...Bb4/4. e5/4...c5
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Winawer variation C17-C19 Winawer variation
4...c5
This is the main line of the Winawer variation. Black has just pushed forward his c pawn with 4...c5, threatening White's pawn chain. White's knight on c3 prevents them from extending the chain by moving a pawn to c3, so this puts White's central pawns in danger.
However, c5 also blocks the escape route for Black's dark-squared bishop. White usually takes advantage of this to regain the initiative with /5. a3/. Black usually trades bishop for knight with 5...Bxc3+ 6. bxc3.
Theory table
Main line 5. a3 Bxc3+ 6. bxc3 Ne7 +
References
French Defence