Tactics Time: Capablanca-Alekhine

Below is a position from the 29th game of the 1927 World Championship match between Capablanca and Alekhine. Capablanca had just played 21 Qb3!

Capablanca-Alekhine

World Championship Match (29)

Clearly, Alekhine would be happy to liquidate his b- & c-pawns for White's b- & d-pawns. However, he did not play 21...c5. Can you see why? (Answer in white-font below the fold.)

Answer:
21 ...       c5?
22 bxc5  bxc5
23 dxc5  Bxc5
24 Bb7   Rc7
25 Nxc5 etc. 
(Golombeck)

23 ... Rb8, accepting the loss of the pawn, is better. 

Capablanca indicated 21 ... b5 22 Nc5 Bxc5 23 bxc5 Ra8 as being the best option.

These are the kinds of little combinations that win and lose games, even down at my level.