Chess Opening Theory/1. e4/1...e5/2. Nf3/2...f5/3. Nxe5/3...Qf6

Black's queen has come lumbering out to poke the e5-knight in the ribs. Should White hold the line, or retreat?

4. d4 is the (old) main line. It temporarily establishes a classical pawn centre (d4 & e4) allowing easy development of the remaining White pieces. If Black now wants to shift the knight from e5, they will have to play ...d6, shutting in the f8-bishop.

4. Nc4 is the Foltys-Leonhardt variation, recommended by Grandmaster Tony Kosten and now considered a more critical test of the Latvian than d4. White knows they can't prevent ...fxe4, so reserves the option of confronting that pawn with a d2-d3 advance when it arrives on e4.

Other knight retreats, 4. Nf3? or 4. Nd3?, allow 4...fxe4 with an attack on the knight, giving Black time to follow up with ...d5 to great advantage.

Wikipedia has related information at Latvian Gambit.