Chess Opening Theory/1. e4/1...e5/2. f4

= King's Gambit = White attacks Black's pawn on e5 with their f-pawn, but the pawn on f4 is attacked and undefended. A gambit is an opening that involves a sacrifice of material (chess pieces, usually pawns) for positional gain. In the case of the King's Gambit, White seeks to tempt Black's pawn away from the centre onto f4, which would give White the freedom to play d4 and e5. The move d4 will not only gain centre space but will also uncover an attack by the c1-bishop on Black's f4-pawn, and Black will have to make further non-developing moves to retain the pawn on f4.

Unfortunately for White, after Black accepts the gambit with:
 * 2...exf4,

White cannot yet play 3.d4. This is because the move 2.f4 also weakens the e1-h4 diagonal, and 2...exf4 weakens it further by controlling the g3 square. Black can therefore respond with 3...Qh4+! and since White cannot play 4. g3 due to the Black pawn on f4, White's king is forced out to the second rank in the opening, leaving Black with a better position (diagram left). 2...exf4 is considered best in high-level chess, but Black must defend accurately.

It's also possible for Black to decline the gambit, with one of the following moves:
 * 2...Bc5 is the usual way of doing so, taking advantage of the fact that Black's e-pawn isn't really threatened (3.fxe5?? gets hit by 3...Qh4+! again). Black makes sure that White won't be able to play d4 or to castle kingside without going to some considerable effort to shift the bishop from its new diagonal.
 * 2...d5 gives the position a different flavour. Normally Black only manages to get in one of the moves e5 or d5 this early in the opening, but since 2.f4 did nothing to prevent 2...d5, why not play it now? Since it attacks the undefended e-pawn, Black will still get to take one of White's pawns if they want to.
 * Black could also reasonably play 2...Nc6, a variation that is rarely explored, or even 2...d6 (The Archaic Defense), which often transposes to either the Fischer Defense of the King's Gambit, or to the 2...Nc6 lines.
 * Even 2...c6!? intending d5 is possible because 3. fxe5?? Qh4+ 4. Ke2 Qxe4+ 5. Kf2 Bc5+, and black has a winning attack, so white is forced to settle with 3. Nf3.
 * 2... Nf6 is the Petrov's Defense of the King's Gambit Declined. This is just fine for black.
 * 2... f5?! is the Panteldakis Countergambit. It is considered dubious with best play, and white gets an advantage, but white must be careful. In the game Kennaugh vs Shirazi, the game continued 3. exf5 e4?! 4. Qh5+ Ke7 5. d3 Nf6! 6. Qg5?!, and black went on to win a wild game.
 * 2... Qh4+?! is the Keene Defense, but this just loses a tempo to 3. g3, and white is already better.
 * 2... Qf6?! doesn't really help black either.
 * 2... c5 is the Mafia Defense.
 * 2... g5? is the Zilbermints Double Countergambit.

Theory table
1.e4 e5 2.f4