Chess Opening Theory/1. e4/1...e5/2. f4/2...exf4/3. Nf3/3...g5/4. h4/4...g4/5. Ne5/5...Nf6/6. Bc4

Black has to do something about the attack by the White bishop and knight on f7. The bishop's diagonal must be blocked at the cost of a pawn with 6...d5.