Setting a trap …

… which White does not fall into. If 21. Qxe5 then Rce8; 22. Qh5, Re1+; 23. Kh2, Qd6+ and Black wins the knight on d1 free and clear.

Bravo! Now it is White's turn to have a dominant knight.

The White knight must leave; this prevents c4 which would re-inforce the knight.

Black now threatens to pile up more pressure with either Rc5 or Rc4, driving the White pieces back.

I did not relish the prospect of the knight coming to f5 any more than I had relished it on d5; therefore, I decided on drastic measures…

This now plunges us into a classic problem endgame: queen and pawns vs. two rooks and pawns, with uneven pawns on both flanks. Probably one of the most complicated endgames to play, with chances for both sides. Theoretically, the two rooks are slightly stronger because they can be two places at once, whereas the queen cannot. Furthermore, two rooks can mate w/o further assistance, but a queen can mate only if the Black king gets trapped behind pawns.

Black goes on the offensive immediately.

And White counter-attacks immediately.

A fateful decision; b4 was also worth considering, with incalculably complicated consequences.