Petrov's Defense: Three Knights Game: what should White play?

In the Petrov's Defense: Three Knights Game, the white e5 is attacked while Black pins the c3 to the white king. Theory recommends Nd3 as the most principled retreat, which scores 54.3% for White across 18K recorded games with an engine evaluation of +0.55. Although Nf3 is the most frequent choice in the database, Nd3 immediately challenges the black b4 and avoids further pressure along the e-file.

Strategic merits of the primary retreats

The retreat Nd3 accounts for 27.6% of recorded games and actively attacks the black bishop on b4. An illustrative continuation shows play proceeding with Nd3 Bxc3 dxc3 Qxe4+ Be2 O-O, where White develops naturally behind an open d-file after the central trade. By contrast, Nf3 is chosen in 45.8% of games (30K recorded) and holds an engine evaluation of +0.81. Following Nf3 O-O Be2 Bxc3 dxc3 Qxe4, White attains a solid position with the bishop pair, though the scoring percentage drops slightly to 50.5%.

Alternative options and tactical pitfalls

White also has access to Ng4, evaluated at +0.61 and played in 6.3% of encounters (4.1K recorded). The illustrative line Ng4 Bxc3 bxc3 Nxe4 Ne3 O-O leads to an unbalanced structure, but White scores only 44.6% from this continuation. Far more dubious is attempting to hold the outpost with f4, which occurs in 17.6% of games (12K recorded). The engine rates this line at an equal -0.03, and White scores just 41.2% after f4 d6 Nf3 Bxc3 dxc3 Nxe4. Finally, the rare retreat Nc4 appears in 2.7% of games, yielding an evaluation of -0.06 in the sequence Nc4 d5 Ne3 d4 Ned5 Nxd5.