Chess Opening Theory/1. e4/1...e5/2. Nc3/2...Nf6/3. Bc4/3...Nxe4/4. Qh5

Black is threatened with 5. Qxf7#. They have nine legal moves that prevent this - surely one of them must allow them to avoid ruin?

• 4...Qe7? and 4...Qf6? allow Nxe4 winning a piece, as without the queen to defend d5 the fork trick has had its prongs bent.
• 4...Qg5? allows 5. Bxf7+ Kd8 6. Nxe4 Qxg2 7. Qxe5 winning a knight and a pawn.
• 4...Qh4? allows 5. Bxf7+ Kd8 6. Qxh4+ winning a pawn and a queen.
• 4...g6? allows Qxe5+ picking up the knight or the rook.
• 4...Ke7?? allows Qxe5# picking up the game.
• 4...d5?? gives the pawn back for nothing as after 5. Bxd5 the threat is renewed.
• 4...Ng5? is met by 5. d4 forcing the knight to e6. Even if knight and king both survive the subsequent complications, White can regain the pawn and enjoy a whopping lead in development.

So by a process of elimination, it has to be 4...Nd6!□, which defends f7 and has the enormous attraction of attacking the undefended c4 bishop.

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1. e4 e5 2. Nc3 Nf6 3. Bc4 Nxe4 4. Qh5

Wikipedia has related information at Vienna Game.