Chess Opening Theory/1. e4/1...e5/2. f4/2...exf4/3. Nf3

3. Nf3 develops a knight and controls the h4 square. This is important to prevent Black from playing Qh4+, exploiting the diagonal White opened with 2. f4.

White now threatens to play d4 and recover the pawn they gambitted with Bxf4. Black would like to hold onto the f4 pawn if possible. There is a huge variety of approaches, but the most common replies are 3...g5, 3...d6, and 3...d5: the classical, Fischer, and Modern defences.

The best way to defend the f pawn is with another pawn, and the only one available is the g pawn. Defending the f4 pawn with a piece is not ideal: 3...Bd6? blocks in Black's d pawn and can be kicked around by e5, likewise 3...Qf6? 4. e5 and the queen can't stay defending the f pawn (4...Qf5? 5. d4 d5 6. Bd3).

3...g5 immediately is the classical variation and main line. As well as protecting the f4-pawn, it threatens to advance to g4 and kick the knight away from f3. Black is not worried about weakening their own kingside because the f4-pawn gives Black all the space on that wing. Any kingside advance from White will run into Black's kingside advance coming the other way.

The most common continuation after 3...g5 is for White to undermine the pawn chain with 4. h4. Then 4...g4 kicks the knight to 5. Ne5, the Kieseritzky gambit. 4...h5 does not work for Black as after White plays hxg4, the h5 pawn is pinned to Black's rook. The next most common is 4. Bc4, which most often leads to the Muzio gambit.

Since f4 is not actually under threat until White has played d4, Black can delay ...g5 and first play a preparatory move, ...h6 or ...d6:

3...h6 is the Becker defence. This prepares 4. d4 g5 5. h4 Bg7. Black's bishop defends their rook, allowing Black to meet 6. hxg5 with ...hxg5.

3...d6, the Fischer defence, is the move by which Bobby Fischer claimed to have refuted the entire king's gambit.^[1] Black would still like to play 4...g5 next move, but first takes the precaution of denying the f3-knight access to e5. This means that after 4. d4 g5 5. h4 g4, 6. Ne5 isn't possible and the knight may have to make an abject retreat to g1. In this way, 3...d6 avoids the Kieseritzky gambit, although as the Kieseritzky gambit actually doesn't score very well for White, Black is usually happy to allow it and play the 3...g5 variations instead.

Black knows that f4 isn't under threat immediately: White has to move their d-pawn first. If Black keeps making threats, they can keep the initiative and disallow 4. d4 or 4. d3 by tempo. Thereby, they prevent the discovered attack on f4 and defend it that way.

Two approaches to do this are to attack the centre with either d5 or Nf6 and keep the initiative that way.

3...d5 is the modern defence. 3...d5 attacks the e4 pawn, so White doesn't have the opportunity to move the d pawn and pick up the f pawn (for example, 4. d3? dxe4 5. dxe4 Qxd1+ 6. Kxd1 Bd6 ⩱).

After 4, exd5, now White can defend the f pawn with 4...Bd6, as in this position Black's d pawn has already moves, and there is no threat of e5 to kick the bishop, or attack the d5 pawn with 4...Nf6. However, this defence is one of the few King's Gambit lines that score better for White than Black.

A variation on the modern defence is to prepare ...d5 with 3...Ne7, the Bonsch-Osmolovsky variation. Then if White trades on d5, Black can get a knight there to defend f4: 3...Ne7 4. d4 d5 5. exd5? Nxd5. However, exd5 isn't forced: White can meet 4...d5 with Nc3 to defend the pawn instead.

3...Nf6 is the Schallopp defence. The knight attacks the e4 pawn. After 4. e5 the knight routes to ...Nh5!, protecting the f pawn. After 5. d4 Black can chip away at the centre with ...d6. A example continuation: 6. Qe2 dxe5 7. Nxe5 Qh4+ 8. g3 Nxg3 9. hxg3 Qxh1 10. Bxf4 ∓. That Black can spend two moves putting a knight on the edge of the board and still win 35% of games (vs 31% for White) is probably a bad omen for King's gambit players.

Alternatively White can respond 4. d3 to defend the e4 pawn. Black has to allow Bxf4 (4...g6? 5. Nxg6 now works, as Black's knight is in the way of 5...Qxg4) to recover the pawn. 4...d5 5. Bxf4 dxe4 6. Qe2 Bd6 7. Bxd6 Qxd6 8. Nbd2 O-O 9. dxe4 Re8 ∞. An interesting try for White is 4. Nc3: protecting e4 and preparing to add an extra attacker on to f4 (4...Nh5? 5. d4 d6 6. Nd5 ⩲).

An alternative way to keep the initiative is to renew the threat to White's kingside.

3...Be7 is the Cunningham defence. Forming a battery between the queen and bishop, it renews Black's the threat of a check on h4. Now if 4. d4?! Bh4+ 5. Kd2 (5. Nxh4? Qxh4+! 6. Kd2) White's king has been forced to move and will be stuck in the centre for some time.

White's correct rebuttal is 4. Bc4. This means 4...Bh4+ can met with 5. Kf1. The usual response to 4. Bc4 is 4...Nf6, but then what's the point of having the bishop on that diagonal?

Minor lines include:

• 3...f5, the Gianutio countergambit. Black thinks White is having a lot of fun giving away their pawns, so offers their own f pawn to deflect White's e pawn. After 4. exf5 d5, Black looks to recover the pawn with Bxf5.

• 3...h5, the Wagenbach defence. Black wants to start storming White's kingside with pawns straight away.

• 3...Nc6, the Macleod defence, is usually followed by 4. d4 d5 to prevent White's pawn coming to d5, 5. exd5 Qxd5 6. Nc3 Bb4.

1.e4 e5 2.f4 exf4 3. Nf3

Wikipedia has related information at King's Gambit.

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