At last White regains his material. But Black is waiting like a trap-door spider.

I'm guessing that White did not see this coming; he probably thought that the e-pawn was adequately secured by the f-pawn. Black's ... Nxe5 does not trade a knight for two pawns--Black will easily get his material back because ...

... of the bishop-skewer attacking the rook on d4 and winning the B on b2 if the rook moves away.

A more stout defence would have been c3 here. White should have acquiesced in the loss of the exchange on d4, retaking cxBd4. That would have lost less material than the text move--2 pieces for a rook and 2 pawns--but, more importantly, it would have left White with a d-pawn to block Black's center pawns. The move as it stands, retreat of the rook to a less favorable square, simply drops 2 pawns with no positional compensation. An alternative was 20. Bd3 or Nd3, BxRd4; 21. QxBd4 or BxBd4, followed by ... e5 and so on; Black's a pawn steamroller in the center will be hard to stop.

Positional assessment at mid-game: Black now has a clear 2-pawn material advantage, a connected passer with queen and rook behind it, control of the half-open c-file, the bishop pair, he stands to win a third pawn outright on the move, and he has no obvious weakness for White to target. His plan for the opening has succeeded: he has cleared the center without loss of a center pawn of his own. Black's plan for the next phase: prepare to set his central pawn mass in motion.

21. ... Bxa3 was weaker. First, because the bishop is much stronger on the long diagonal to help support the coming central pawn storm. Second, because the rook on the 2d rank is about to wreak havoc behind the White lines. 20. Rd4-d3 was clearly off the mark; if the rook was to have retreated, then Rd2 would have held the crucial c2 pawn. As it stands, Black now has two connected passers in the center. This should be sufficient to force a win by direct advance of the d- and e-pawns.

It is understandable that White should try to avoid further loss of material by hanging onto the a3 pawn here. However, he has under-estimated the threat posed by the Black rook on c2: the White bishop is now irretrievably lost.

There is only one square to which the queen can move to hold the bishop, and that is f3; whereupon Black continues ... d4 forcing the White queen away, and the rook picks up the bishop for free.

There was no saving the bishop anyway, so White abandons the bishop to his fate and tries to break up the Black king position.

If 25. h5, then ... g5 and the White attack is halted in its tracks.