KEG: Showalter overlooks mate in one and gets checkmated on move 18. While this is shocking, Showalter at least had the consolation that his horrendous blunder probably did not change the outcome since Maroczy had a won game even before the fatal mistake.
1. e4 e5
2. Nf3 Nf6
A reasonable alternative to the more usual 3. Nxe5
4. Bd3 d5
5. Nxe5 Bd6
6. 0-0 0-0
As played by Brody in his loss to Marshall five days earlier in this tournament. Best is probably 7. Nd2.
Marshall's move in his game with Brody. It is certainly better than the tempo-losing 7...Nf6 recommended by Rosenthal in the Tournament Book. Best was 7...BxN.
Much better than 8. Qc2 as played by Brody against Marshall.
8...NxN was better. Little by little, Showalter is getting into trouble.
9. dxB NxN
10. bxN dxc4
11. Bxc4 Be6
Also bad was 11...Bf5 which--as analyzed by Rosenthal--would have been followed by 12. Ba3 Re8 (12...QxQ was better here, though White would still be much better) 13. Qb3 with advantage to White.
Best for Black here was to trade Queens. After the text, Maroczy has or is close to a strategically won game.
12. BxB fxB
13. Ab3 Qc8
Even better was 14. Rb1.
This was Showalter's best chance at counterplay.
15. Radi b6
16. Rd6 Re8?
Weak. 16...Nc6 was clearly best. Maroczy's position is now overwhelming.
This was the position after 17. Qa4 (and right before Showalter blundered into a one-move checkmate):
As Rosenthal notes in the Tournament Book, best for Black now was 17...Na6 18. Rfd1 (or 18. Qg4) Nc7. According to Rosenthal, the game would then be even. But this is nonsense. White would now play 19. Qg4 (or even 19. Bc1) with a powerful attack that would probably have been sufficient for victory.
18. QxR mate