Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nf6/3. Nxe5/3...Nxe4

Black has entered an inferior variation of Petrov's Defence. This position is usually only seen among beginners. White's response is 4. Qe2, which threatens the black knight on e4.

If black decides to move the knight away, perhaps with Nf6, white will respond with Nc6+! which threatens black's queen and gives discovered check from the white queen down the e-file. As black must move their king or block the check, the queen will be lost on the next turn with Nxd8 or Nxe7 if black moved Qe7 to block the check.

Thus, black's best move is not to move the knight. Attempting to protect it with d5 or f5 usually ends poorly, as the knight is pinned and white can pile on attackers with f3 and d3.

However, after the planned Qe2-Qxe4, white's queen will be undefended, which black can exploit via the sequence 4.Qe2 Qe7 5.Qxe4 d6, winning the piece back at the cost of the d-pawn.

Ultimately this line is a pawn sacrifice for which black obtains some compensation. It is unclear whether the compensation is objectively sufficient, but it offers decent practical chances, especially at faster time controls.

Theory table
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1. e4 e5 2. Nf3 Nf6 3. Nxe5 Nxe4