njchess: I got this puzzle. However, like many before have pointed out, it wasn't obvious because there was more than one good choice. Bxd5 probably transposes, but I chose Bxg7 because it is more forcing, opens up Black's king side and, over the board, is more dramatic and maybe unexpected. From there, White just kept sacrificing material to keep the attack going.
<DoubleCheck How did black get into a losing position before whites 20th move?
Answer: Most likely with that questionable 19...h5??
The position after 18...Nxd5 is a small space/development advantage for white but nothing overly significant until 19...h5?? was played>
7. ... Be7 is tame, but playable. I prefer Bd6 since it attacks the center and forces White to commit to defending the e5/c5 squares. At this point, neither of Black's bishops are on terribly good squares, but, in Black's defense, it is only the seventh move.
White responds with the aggressive 8. Ne5 (Qc2 is also playable and could transpose with Ne5). Black plays 8. ... 0-0 in response. Normally I'm not one to criticize a safety first mentality, but already White is building a dangerous center. I think the need to counter in the center outweighs castling at this point so, I would have played 8. ... Nfd7.
10. ... Na6? An odd, if unnatural move by Black that has the knight either going to c5 (most likely), c7 (to protect d5, but way too slow given the position) or even b4, but misses the point. White wants to control d5, preferably by occupying it. Nbd7 is stronger, especially in conjunction with Bd6.
As when Black played Be7, White responded aggressively with Ne5, here again, White responds aggressively to 10. .. Na6 with 11. e4!? Given that Black's queen side knight is out of position, this move makes sense. White recognized that Black was making positional moves and sought to bring direct pressure in the center before Black was ready. Other moves were the quieter a3, Rc1 or even e3.
Black ignores the the threat to the center and plays 11. ... Rc8 which is thematically consistent with his other moves up to this point. Black doesn't have really any better options. Bb4, Qc7, Re8 or even dxc4 were all possibilities.
12. Re1 is fine in the face of the impending Bb4. Also playable was Qe2 followed by f3, but I like the game move better since it doesn't weaken White's king side at all or hinder the bishop on g2.
Black plays 12. ... Bb4 seeking get rid of his bad bishop and to weaken the c4 pawn. Since White has the knight on e5, it doesn't worry White in the least, but Black has nothing better.
13. exd5 cxd5 14. a3 Bxd2 15. Qxd2 is a natural progression. 15. ... Qc7 16. Rac1 dxc4 17. Rxc4 Qe7 is not as accurate as 15. ... Qe7 16. Rac1 dxc4 17. Rxc4 which leaves Black with the initiative. Its a small error, but combined with previous moves (e.g. Na6), it adds up. With Black's dark squared bishop off the board, f3 is an alternative to Rac1. However, I like the game move better for reasons given above.
Up to this point, if you were using a computer, it would probably tell you that the position has been and still is essentially even or slightly in White's favor. However, all of White's minor and major pieces are active. His central pawn can advance, and thanks to Black's inaccurate play, he still possesses the initiative to advance it. Moreover, Black's queen is unprotected and indirectly threatened by the White's rook on the half open e-file.
18. d5! shatters Black's position. Black really has no good reply. Sooner or later, Black will either recapture with the e6 pawn losing his queen or have to recapture with the queen on f6 losing it in the process. I suppose Black chose 18. ... Nxd5 to avoid dropping his queen to the rook, or maybe he thought that he might have enough material to compensate for losing the queen on f6. Or maybe, he just overlooked 19. Ng4! In any case, no matter what Black does, White has a winning game though he still needed to play accurately to win.
19. ... h5 was maybe not the best. f5 might have put up more resistance in that he would not have been mated.