2...Bc5
This is an offbeat but playable alternative, as played (for example) by former world champion Capablanca against Ilya Kan at Moscow 1936. Now 3.f4 d6 leads to the King's Gambit Declined. Weak is 3.Qg4 Nf6! 4.Qxg7 Rg8 5.Qh6 Bxf2+ when Black had a large advantage in Tsikhelashvili–Karpov, USSR 1968, since 6.Kxf2?? Ng4+ would win White's queen. Another offbeat possibility is 3.Na4, when 3...Bxf2+!? 4.Kxf2 Qh4+ 5.Ke3 Qxf4+ 6.Kd3 d5 leads to wild complications, as in the famous game Hamppe–Meitner, Vienna 1872. However, the quiet 3...Be7 leaves Black with a good game.
Instead of these third moves, White usually continues with 3.Nf3, when 3...Nc6?! (transposing to the Three Knights Game) 4.Nxe5! Nxe5 5.d4 Bd6 6.dxe5 Bxe5 7.Bd3 leads to a large advantage for White. Stronger is 3...d6! Then 4.Na4 Nd7 5.d3 Ngf6 6.Be2 0-0 7.0-0 c6 8.Nxc5 Nxc5 9.Ne1 Ne6 10.c3 d5 is about even. The main line runs 4.d4 exd4 5.Nxd4 Nf6 6.Bg5 (6.Be2 d5 7.e5 Ne4 8.0-0 Nxc3 leads to equality) h6 7.Bh4 0-0 8.Nb3 and now de Firmian in MCO-15 gives 8...Bb4 9.Bd3 Re8 10.0-0 Bxc3 11.bxc3 g5! 12.Bg3 Nxe4, when Black's "chances are at least equal".
Read more about this topic: Vienna Game