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Chess Opening Theory/1. e4/1...e6/2. d4/2...d5/3. Nc3/3...Bb4/4. e5/4...c5/5. a3
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Winawer variation C17-C19 Winawer variation
5. a3
This is the main line of the Winawer variation. Black's bishop is pinning White's c3 knight, and their c5 advance has just put additional pressure on White's center. At this point, White has chosen to regain the initiative by kicking out the bishop with 5. a3.
Black usually takes the knight with /5...Bxc3+/, because moving /5...Ba5/ leaves the c5 pawn undefended and open to White's 6. dxc5.
Theory table
Main line 5. ... Bxc3+ 6. bxc3 Ne7 + Retreat variation 5. ... Ba5 6. b4 cxd4 7. Nb5 Bc7 +
References
French Defence Winawer_Variation:_3.Nc3_Bb4