Italian Game, Rousseau Gambit

The Rousseau Gambit is a chess opening that begins with the moves:
The gambit is named after French chess master Eugène Rousseau. White can decline the gambit by supporting the e-pawn with 4.d3. The resulting position is similar to a King's Gambit Declined with colours reversed, and White's aiming at Black's weakened. Black will have trouble castling kingside, and Ng5 is a likely threat. White's position is better, but still requires careful play.
Key themes for White are to attack Black's kingside and to avoid attempts by Black to the position. Exchanges involving White's are particularly suspect.

White responses

Gambit Declined: 4.d3

White can decline the gambit and wait to capture the f-pawn. 4...Bc5 transposes to the Lucchini Gambit.

Gambit Accepted: 4.exf5

White still has a good game after the inferior 4.exf5, but the position is less clear. Black usually plays 4...e4, which White may meet by 5.Nd4 Nf6 6.Nxc6.

4.d4!

White gets a clear advantage with 4.d4!:

4...fxe4 5.Nxe5 d5 6.Bb5 Ne7 7.0-0 a6 8.Bxc6+ bxc6 9.f3 Bf5 10.Nc3 +/−.
4...d6 and now:
* 5.Ng5 Nh6 6.d5 Nb8 7.Nc3 f4 8.h4 Bg4 9.f3 Bd7 10.g3 fxg3 11.f4 +/−.
* 5.dxe5 and now:
** 5...fxe4 6.Qd5 Qe7 7.Bg5 Be6 8.Qxe4 +/− de Rivière–Anderssen, London 1862.
** 5...dxe5 6.Qxd8+ Nxd8 7.Nxe5 fxe4 8.Bd2 Bd6 9.Bc3 +/− Löwenthal & Medley vs. Morphy & Mongredien, London 1857.
4...Nf6 5.dxe5 Nxe4 6.0-0 Bc5 7.Nc3 Nxc3 8.bxc3 h6 9.Nd4 g6 10.Nb3 +/−.