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

Black has put a pawn on g5 to defend f4. Left to her own devices, she could reinforce this pawn with ...h6 and ...Bg7 and turn the f4-pawn into a mighty fortress. If White wants to throw a spanner in the works, the spanner must be thrown immediately, with This move catches Black on the hop, 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 will be forced to advance to g4. "Forced" may be too strong a word, because the pawn had ambitions of reaching g4 and kicking the knight as soon as it took off from g7, but there's another crucial idea behind 4.h4: the Black queen can't now take advantage of the knight's disappearance from f3 to deliver check on h4. 4. d4 and 4. Nc3 are less common moves, based on a similar idea to 4.Bc4.
 * 4. h4!
 * 4. Bc4 is a move that puts a different sort of question to Black. The g-pawn can now advance to g4 and kick the knight if it wants to - but White plans to carry on developing and allow it to be captured! Is Black prepared to fend off the blistering attack that will ensue, or is she going to leave the g-pawn where it is and reinforce it with ...Bg7 and ...h6?

Theory table
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'''1.e4 e5 2.f4 exf4 3. Nf3 g5'''