Chess Opening Theory/1. e4/1...e5/2. Nc3/2...Nf6

As White, you still have designs on playing f4, but Black with their knight sortie has challenged your control of d5. Should you fight back, or ignore this distraction?

3. f4: White has had enough of Black's e-pawn and feels an open f-file would be useful in their later attack. This is a King's Gambit but with Nc3 added so f4 becomes a real threat to the e5 pawn.

3. Bc4: White again restrains themselves from the committal f4 and places another piece where it controls d5. With a little co-operation from Black, White may now head towards one of the most terrifying opening positions yet discovered...

Less critical alternatives are:

3. a3: the Mengarini Variation, preventing Bb4, but this bishop has at least one other good square.

3. d3: Having challenged Black to some old-fashioned fisticuffs, White sadistically transposes to a solid King's Indian Attack.

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1. e4 e5 2. Nc3 Nf6

Wikipedia has related information at Vienna Game.