Chess Opening Theory/1. d4/1...f5/2. Bg5/2...h6/3. Bh4

This retreat appears to lose the bishop, but it doesn't! If 3...g5 4.Bg3 f4?!, 5.e3! h5 6.Bd3! gives a crushing attack.

Theory table
g5  f4?!

4.Bg3 5.e3