Chess Opening Theory/1. e4/1...e5/2. d4

Center Game
The move 2. d4 by White smashes open the center by attacking Black's king pawn and allows White to develop his pieces with very great speed by opening up the d-file for the queen and bishop.

Black's responses
The most common response is for Black to accept the Centre Game and take the pawn with 2...exd4, which White will retake with the queen. Although no pawn material is lost here, White will lose tempo in order to save the queen from Black's Nc6 knight attack. This gives Black a lead in development, which brought this opening out of favor historically by the early 20th century although the opening has been explored later from the 1980s.

Defending the e-pawn instead with 2...d6 leads to a transposition of the Maróczy Defence stemming from the Pirc Defence (1. e4 d6 2. d4 e5). The Maróczy Defence may then lead to the Philidor. However, White can choose not to transpose here since Black's 2...d6 gives White the chance now to aggressively capture the e-pawn with 3. dxe4 rather than defend their d-pawn with the Philidor's 3. Nf3 knight defence. Once the pawns are traded off, White can force off the queens as well which denies Black the ability of castling – possibly leading into an early endgame.

Instead of the normal 2...exd4, a countergambit can be tried with 2...f5?!, but this is quite a dubious move and is only reserved for unprepared players. 2...f5 gives White a good game after the threat of Qh5+ can't be stopped (the bishop can move and give a square to the king on f8), as after 3. exf5 Nf6?? loses to 4. e5!, and Black has two choices: letting a very dangerous Qh5+ happen or giving up the knight. Once again, unprepared opponents may be surprised by this and play recklessly, leading to possible, but rare wins by Black.

Theory table
'''1. e4 e5 2. d4'''