Chess Opening Theory/1. e4/1...Nc6/2. d4/2...d5/3. Nc3/3...a6/4. exd5/4...Nb4/5. Bc4

= Nimzowitsch Defence = White's pawn on d5 is now defended and attacked twice. After /5...Nf6/ there is no good way for White to defend the pawn a third time and it will fall. 5...Bf5 seems like a good move as it forces White's bishop to move again with 6. Bb3, but after 6...Nf6 7. Bg5 Nbxd5 8. Nxd5 Nxd5 9. Qf3 Be6 Black's game is worse.

Theory table
.

'''1. e4 Nc6 2. d4 d5 3. Nc3 a6 4. exd5 Nb4 5. Bc4'''