Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nf6/3. Nxe5/3...d6

White's knight is under attack and must be moved, then it is safe for Black to pick up the pawn.

4. Nf3 is the overwhelmingly common choice, and Black retakes the pawn with 4...Nxe5. This reaches the main junction of the Russian game where there are several theoretical continuations; the main line is 5. d4, the classical attack.

4. Nd3!?, the Karklins-Martinovsky variation has been tried occasionally, notably played by Magnus Carlsen in the 2018 World Chess Championship against Fabiano Caruana.^[1] Temporarily blocking the d-pawn, White's knight will eventually make way for it by rerouting to f4. 4...Nxe4 5. Qe2 Qe7 6. Nf4 Nf6 7. d4 Qxe2+ 8. Bxe2 Nc6 ⩲.

4. Nc4!?, the Paulsen attack, avoids blocking the d-pawn, but is no more effective than 4. Nd3 according to the engine. The knight usually ends up falling back to e3.

4. Nxf7?! is the Cochrane gambit, a double-edged but dubious gambit. White sacrifices their knight to drive Black's king out into the open and keep the initiative. After 4...Kxf7 5. d4 Nxe4? is a mistake due to 6. Qh5+ g6 7. Qd5+ Kg7 8. Qxe4 ±.

1. e4 e5 2. Nf3 Nf6 3. Nxe5 d6

Wikipedia has related information at Petrov Defence.

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.