1.Nf3 f6 2.e3 d5

The critical line starts with 3.Ne5 fxe5/3.Ng5 fxg5 (black can't allow 4.Nf7) 4.Qh5 g6 (only move) 5.Qe5. Now the best move for black is 5...Be6 (5...e6 6.Qxc7 and white should be winning).

Note that after 3.Ne5 (or 3.Ng5) black can try 3...Bg4. This move is better in the 3.Ne5 case, since 4.f3 fxg5 crushes the Bg4.

Other lines:

3.Nd4 Bg4 (or 3...b5) 4.f3 (or 4...Qf3) 4...Bf5 5.Ne6 Bxe6 6.Nc3 (or 6.d4) and black can choose 6...a6, 6...b5 or 6...c6,

3.Nc3 b5 4.Bd3 (or 4.Ng5 fxg5 5.Qh5 g6 6.Qe5 e6 7.Qxc7 Nf6 8.f3 Bh6) 4...g6 5.Nd4 e5,

3.Bd3 g6 4.Nd4 Bg4 5.Qf3 (or 5.f3).