Transposing to the Two Knights Defense.
White chooses the Perreux Variation, which forces either 5....Ne5 or 5....d5 to block the attack at f7. The players now enter the main line of the Perreux Variation, where White is able to weaken Black's pawns in an otherwise equal position, leading to chances for both sides.
In his earlier game against Torre, Marshall took with the Queen, leading to a more simplified position after 12....Qxe6 13.Qxe6+ fxe6. Perhaps he feared an improvement on Santasiere's part. Keeping Queens on the board, though, does give Black chances to exploit his centralized pieces. The Queen at f5, for example, prevents the natural development of White's Bishop to either f4 or g5.
Not 13.Re1 Bb4! -- a tactic that Marshall successfully employed in his game with Torre.
In the game Blackburne--Schlechter, Ostende 1905, play continued 13....a6 14.Rad1 Bd6 15.f3 Qe5 16.g3 Bc5 17.Bxc5 Qxc5+ 18.Qf2 Qxf2+ 19.Kxf2 Rd6 20.Rfe1 Rb6 21.Na4 Rc6 22.Nc3 Rb6 1/2-1/2. Here Marshall tempts his young opponent to capture the loose a-pawn with 14.Bxa7, when the wayward Bishop might eventually be trapped with b6 or allow Black free reign on the Kingside for an attack. White should have considered the capture, however, since there is no immediate way for Black to exploit the move. If 14.Bxa7 b6? 15.Qa6+ Kd7 White will at least secure three pawns and an attack for the Bishop, and all speculative Kingside attacks appear to be repulsed (e.g.: 14....Bxh2+? 15.Kxh2 Ng4+ 16.Kg1 Qh4 17.Qxe6+ Rd7 18.Rfd1 +-).
Likely Black planned to meet 14.Bxa7 with the simple 14....Qe5 15.Qxe5 (Forced due to the threat of mate at h7, and if 15.f4? Qxe2 16.Nxe2 b6! traps the wayward Bishop) 15....Bxe5 16.Be3 (The Bishop must escape being trapped with ....b6) 16....Bxc3 17.bxc3 and while Black has surrendered a pawn, he has also created an imbalanced situation where both players have objects of attack. Santasiere would likely then not have escaped with his cherished draw, though a master equal to Marshall may have been able to turn the pawn to account.
The exchange of pieces seems to aid Black's development. But White could not retreat with 15.Nd4? since Black has the surprising reply 15....Bxh2+! 16.Kxh2 Qe5+ 17.Kg1 Rxd4! =+ and if 17.f4? then 17....Qxe3! 18.Qxe3 Ng4+! wins for Black. It is surprising that such simple positions contain such surprising tactics, and I must confess that I missed this line until it was pointed out by a friend!
Black now leads in development and has a better grip on the important d-file. White must therefore oppose Rooks, but the resulting exchanges favor Black whose remaining pieces are more centralized. The position remains balanced, though, because of the weakness at e6. But Black can do quite well here if White is not careful.
Probably not the best idea here if White wants to try for an advantage. If White wants to exchange any pieces, he wants to exchange the Queens, since the combination of Rook and Bishop is better than Rook and Knight due to the long-range scope of the Bishop. Also, White can try to organize an attack on the queenside, especially since Black has weakened his castle there by 14....a6 while White's kingside catle position is perfect. Best, therefore, is probably 18.c4! with the idea of playing b4-b5 or even c4-c5-c6! if Black allows it. Play is well balanced, but at least White will have a plan. The way Santasiere plays the position, his only plan seems to be to simplify as much as possible and run for the draw. You cannot run for a draw against a swindler like Marshall, who fought for every point.
The other advantage of 18.c4! is that the Knight is kept out of d5, at least for the moment, and White will have a better position than in the game after 18....Rd3! (threatening to put pressure on the Bishop with Ng4 and Qe5) 19.Rd1!? (Better is probably 19.f3 or 19.h3 to protect all the light squares, followed by b4-b5 to attack on the wing, though White will then have weakened his own castle position and will invite a pawn break by either e5-e4 or g5-g4 by Black so there is roughly equal play all around) 19....Rxd1+ 20.Qxd1 = etc.
Suddenly the Knight springs forward, and the supposedly weak pawn at e6 supports it beautifully in the center. Black's pieces are well placed and he avoids the exchange of Rooks so as to keep pieces on the board for an attack. Often, the combination of Knight and Queen is better than Bishop and Queen because the Knight can attack squares of both colors and cooperates well with his Lady in the attack. Black likely has a slight edge here.
An unfortunate necessity, since 22.Qxe3 Qxe3 23.Rxe3 Rd1+! 24.Kh2 Rd2! 25.Rxe6 Kd7! leads to a winning endgame advantage for Marshall. Though Black retains clear superiority in the heavy piece ending, there is no immediate breakthrough and the game enters a long maneuvering stage. In what follows, however, we see the clear superiority of the Grandmaster over the Amateur: while Santasiere sometimes appears to be killing time by harrassing Black with his Queen, Marshall is always trying to gain more space and to improve the position of all his pieces and pawns.
22....Rd3 23. Qf2 g6 24. Re2 Rd1+ 25. Kh2 b5 26. a3 h5 27. Qf4 Qb1 28. Qf8+ Kb7 29. Qf3+ Kb6 30. Qf6 Qe4 31. Qf4 Qd5 32. h4 e5 33. Qf8 Qc4 34. Qf6+ Rd6 35. Qf2 Qe4 36. Re1 c6 37. Kg1 Rd3 38. Kh2 a5 39. Kg1 a4 40. Kh2 Rd6 41. Kg1 Kc5 42. c4!?
An interesting attempt to gain some play and to prevent Black's King from walking freely into the Queenside via Kc4-b3. If now 42....Kxc4? 43.Rc1+ wins.
This allows White to convert to a clearly winning King and pawn ending. But not much relief was to be found in 50.Rf3 Rd7, though Rook endings always take a while to win.