Planning: Capablanca - Alekhine, St. Petersburg 1913

Continuing with Capablanca's games 'to be studied', this game has me baffled. In the diagrammed position, where the players have just exchanged the dark squared Bishops on e7, Capablanca played 18.Be4. He commented,

This move I considered a very long time.It looks very simple and inoffensive, yet it is the foundation of the whole attack against Black's position. The fact is that the Bishop is doing very little, while the Black Knight at d5 is the key to Black's defense, hence the necessity of exchanging the almost useless Bishop for a most valuable Knight. It would take a good many lines to explain this move properly, and then I might not be clearly understood, so I leave the student to work it out by himself.

St. Petersburg 1913
Alekhine, Alexander

Capablanca, Jose Raul
(After 17...Qd8-e7(xB))
[FEN "r4rn1/pp1bqpkp/4p1p1/3nN3/3P4/3B1N2/PP1Q1PPP/2R2RK1 w - - 0 18"]

Looking first at the alternatives, White would like to play 18.Rc7 but there is the small problem of 18...Nxc7. Another idea is 18.Ng5 attacking the f-Pawn twice. Capturing it by 19.Ngxf7 isn't good because of 19...Rxf7, but Black must guard against the possibility. It is interesting to note that after 18.Ng5, 18...f6 doesn't work because of 19.Nxh7 Kxh7? 20.Nxg6. Another idea is 18.Qa5, eyeing c7, but Black has 18...b6 19.Qa6 Nb4.

Alekhine played 18...Bb5. What did Capablanca intend if Black plays the logical 18...Ngf6? The Bishop is attacked, so White can exchange it, defend it, or retreat it.

• If 19.Bxd5 Nxd5, the Knight which was doing little on g8 has taken the place of the 'most valuable Knight' on d5. Neither 20.Ng5 f6 21.Nxd7 Qxd7 22.Ne4 b6, nor 20.Nxd7 Qxd7 21.Ne5 Qd6, looks convincing. Why should White trade the powerful Knight on e5 for the Bishop blocked by its own Pawns?
  
  
• Perhaps White intended 19.Ng5 Nxe4 20.Nxe4, followed by 21.Nc3 (not 21.Ng4 f5), planning to exchange a pair of Knights.
  
  
• Or perhaps 19.Bb1, planning 20.Ng5 h6 21.Ne4 exchanging a pair of Knights followed by Be4xd5.

The game continued 19.Rfe1 Qd6 (19...Ngf6 20.Bxd5 Nxd5 21.Rc5) 20.Bxd5 exd5 (20...Qxd5? 21.Rc5) 21.Qa5. Capablanca:

Now the square c7 is controlled by White, and this decides the game. Should Black attempt to protect it by} 21...Bc6, he will soon lose a Pawn through 22.Nxc6 as White will be able to bring up enough forces and win either the a- or c-Pawn. It should be noticed that Black's weakness throughout the middle game is his inability to command the Black squares.

Alekhine played 21...a6, and after 22.Qc7 Qxc7 23.Rxc7 h6 24.Rxb7, White's extra Pawn with a Rook on the seventh rank was enough to win the game. That is impressive enough, but what would the outcome have been after 18...Ngf6?

To play through the complete game see...

Jose Raul Capablanca vs Alexander Alekhine, St Petersburg m (01) 1913
http://www.chessgames.com/perl/chessgame?gid=1011878

...on Chessgames.com. Perhaps the many comments on the game there will shed some light on this position.