[<< wikibooks] Chess Opening Theory/1. d4/1...Nf6/2. c4/2...e6/3. Nc3/3...Bb4/4. a3/4...Bxc3+
= Nimzo-Indian Defence, Sämisch Variation =
Black takes the knight with 4...Bxc3+. White must recapture with 5. bxc3. White will have the bishop-pair and a large center, but Black will have counterplay against the weak c4-pawn.