Chess Opening Theory/1. e4/1...Nc6/2. d4/2...d5/3. Nc3/3...dxe4/4. d5/4...Ne5

Nimzowitsch Defense
Following 4...Ne5, White could recapture the gambitted central pawn with 5. Nxe4, but following 5...e6! Black is guaranteed good development and almost certain equality.

A more critical response is 5. Bf4, threatening the knight on e5. 5...Ng6 is forced, due to the fact that 5...Qd6?? loses to 6. Nb5! Nf3+ 7. Qxf3!!, and after 6. Bg3, 6...f5 is critical, although largely unexplored.

White can avoid these lines with 5. Qd4, also forcing 5...Ng6. This line is more aggressive, and Black must be careful, but Black can probably equalize.