Marshall Attack, Modern Main Line

The Modern Main Line is the central position of the Marshall Attack, sometimes called the Marshall Gambit, in the Ruy Lopez or Spanish Game. It arises after 11...c6 12. d4 Bd6 13. Re1 Qh4 14. g3 Qh3. The queen's excursion first provoked g3, loosening the light squares around White's king. It now settles on h3, where it cooperates with the bishop on d6 and the knight on d5. White keeps the extra pawn, but the knight on b1 and bishop on c1 are still undeveloped.

Black's immediate idea is ...Bg4. The bishop would then support ...Bf3 and ...Qg2 mate, so White's queen would be tied to guarding f3. That would leave the rook on e1 unsupported, and Black would follow with ...Rae8 to take over the e-file. White's fifteenth move is chosen with this plan in mind.

The older main move is 15. Be3. It closes the e-file in advance and clears the c1 bishop out of the way. The knight can then reach d2 and head toward the kingside without shutting the bishop in, which 15. Nd2 at once would do. The drawback is that the bishop has little to do on e3. When Black later advances ...f5, White frequently has to reply f4 to prevent ...f4, and the bishop then becomes a target for the knight on d5 and a rook on e8.

15. Re4 is more direct. The rook rules out ...Bg4 and threatens Rh4 to evict the queen. A database comparison on Wikibooks found that 15. Re4 outperformed 15. Be3 even after adjusting for the higher ratings of its users. It also noted that, as of 2018, White's last 13 wins against grandmasters had all come in lines other than 15. Be3.

Online practice follows a different pattern. 15. Qf3 is the most frequent choice on Lichess, yet Black wins close to two-thirds of those games. 15. Be3 and 15. Re4 come next in popularity and give White noticeably better results, with 15. Re4 the most successful of the common options. 15. Bxd5 and 15. Qd3 are also played, while the immediate 15. Nd2 scores very poorly for White. The engine assesses the position as balanced and favors 15. Qe2, 15. Re4 and 15. Qd3. Across all continuations, Black scores well from this position online.