Chess Opening Theory/1. e4/1...e5/2. f4/2...exf4/3. Nf3/3...g5

King's Gambit Accepted

	a	b	c	d	e	f	g	h
8									8
7									7
6									6
5									5
4									4
3									3
2									2
1									1
	a	b	c	d	e	f	g	h

Common moves:
4. h4 - Main Line (Paris Attack)
4. Bc4 - Classical Variation
4. d4 - Rosentreter Gambit
4. Nc3 - Quaade Gambit

Black has put a pawn on g5 to defend f4. Left to his own devices, he could reinforce this pawn with ...h6 and ...Bg7 and turn the f4-pawn into a fortress. However, the most common and forcing threat is for Black to play ...g4, flush White's knight away, and play Qh4+ to force White's king to go uncastled. White has two direct defenses and two indirect defenses to defend against ...g4.

4. h4! is the most mainline direct defense against ...g4, as now the Black queen cannot make use of h4 with check. This move catches Black off guard, because g5 is attacked twice and defended only once, and 4...h6? doesn't protect it (5.hxg5 hxg5?? 6.Rxh8 is a disaster). 4...gxh4 would leave Black with three isolated pawns on the kingside. So the pawn advances to g4 and kicks the knight.

4. Bc4 is the most commonly-played direct defense against ...g4, vacating the f1 square for the White king to hide after Qh4+. Black could reinforce their g-pawn with ...Bg7 and ...h6, but much more common is ...g4, leading to some of the most famous variations such as the Muzio Gambit, Lolli Gambit, and Salvio Gambit. White can either sacrifice their knight and develop, sacrifice the bishop on f7 to lure out Black's king, or play Ne5 and coordinate on f7.
4. d4 is an indirect defense against ...g4, opening up the dark-squared bishop to recapture on f4. After ...g4, White can either play Nc3 or Bxf4 and give up the knight for an attack, or play Ne5, allowing ...Qh4+, but after g3 fxg3, Qxg4 saves the day. If Black goes for the rook with g2+, the position is equal! In fact, White gets an attack so extreme that it's very easy for Black to blunder the game away.
4. Nc3 is an indirect defense against ...g4, seemingly ignoring the threat on ...g4 but presenting a very strong counterattacking idea; after Ne5 Qh4+ g3 fxg3 Qxg4 g2+ Qxh4 gxh1=Q, White can go for a Scholar's Mate, and one day, the knight on c3 will jump to d5 with devastating effect.