Note (added 6/22/10): Variation B has been extensively augmented to cover 11. cxb6, which soon leads to Black's sacrifice of a rook.

Note (added 6/27/07): A discussion of some of the analysis presented below may be found at Pete Tamburro's "Openings for Amateurs" forum. I have been meaning to incorporate some of that discussion into my analysis, but as I still haven't done it, readers are encouraged to visit Pete's site (which may require creation of a user login). Here is the link to the applicable thread.

Now to the analysis...

1. e4 Nf6 2. e5 Nd5 3. d4 d6 4. c4 Nb6 5. f4

The Four Pawns Attack of the Alekhine Defense, which is believed to give White an advantage. But as Black's first four moves seem quite sane, perhaps there is a hidden resource for undermining the White pawn center before it can be consolidated.

5...g5

This move has not done well in practice, but I believe it is viable if followed up correctly. Its purpose is to undermine White's pawn chain by pressuring its base at f4, while enabling Black's bishop to develop effectively at g7. Though 5...g5 weakens the h6 square and the b1/h7 diagonal, White's pawn advances seem at least as compromising, having created holes at d3, d4, e3 and e4. And as will be seen, the published refutations are readily sidestepped by Black.

Since many of the positions arising from 5...g5 are too complex to permit the enumeration of all plausible alternatives, I will focus on what appear to be the most critical lines.

Sharp tries by White:

A. 6. fxg5
B. 6. exd6
C. 6. d5

A. 6. fxg5

6...dxe5 7. dxe5 Qxd1+ 8. Kxd1 Nc6 9. Bf4 Bg7 10. Nf3 Bg4, and Black regains the pawn while remaining ahead in development.

B. 6. exd6

6...cxd6

Theory only gives 6...gxf4 7. dxc7 Qxc7 8. Nc3 which is clearly better for White, given the weakness of the f4-pawn.

7. fxg5

The pawn sacrifice has exposed White's d-pawn to pressure along the a1/h8 diagonal, accentuated by the tempo White has just expended. Moreover, the extra pawn's being doubled means that White's kingside pawn majority does not constitute a potential passed pawn.

7...Bg7 8. Nc3 Nc6 9. Be3 d5 10. c5

10...e5

After 10...Nc4 11. Bxc4 dxc4 12. Nge2, Black seems to have inadequate compensation for the pawn.

11. cxb6

More challenging than 11. dxe5 d4 12. cxb6 O-O, which leaves White with many weak pawns.

11...exd4 12. Nb5 dxe3 13. Nc7+ Kf8 14. Nxa8 Bxb2 15. Rc1

The most viable means to prevent 15...Bc3+. At this point, 15...Bxc1 16. Qxc1 would seem to leave Black without an effective forcing continuation.

15...Bf5 16. Nc7 axb6 17. Nb5 Qa8

This seems more thematic and better than 17...Qe7. This sequence is reminiscent of another variation where Black's queen occupies a freshly opened a-file: 1. e4 Nf6 2. e5 Nd5 3. d4 d6 4. c4 Nb6 5. f4 dxe5 6. fxe5 Nc6 7. Be3 Bf5 8. Nc3 e6 9. Nf3 Be7 10. d5 Nb4 11. Nd4 Bg6 12. a3 c5 13. Nxe6 fxe6 14. axb4 cxb4 15. Na4 O-O 16. Nxb6 axb6 17. Rxa8 Qxa8 18. d6 Bd8 19. Be2 b3.

18. Be2 Qa5+

18...Qxa2 19. Nf3 Qa5+ 20. Kf1 Bxc1 21. Qxc1, and White's queen may cause trouble along the a1/h8 diagonal. The text intends to avoid this by exchanging queens in a few moves.

19. Kf1 Qxa2

Black would not fare well after 19...d4 20. Rxc6 bxc6 21. Nxd4; 19...Qd2 20. Qxd2 exd2 21. Rd1 Bc1 22. Nf3; or 19...Bxc1 20. Qxc1 Qd2 (20...Qxa2 21. Qxe3) 21. Qxd2 exd2 22. Nf3.

20. Nf3

20. Nc3 doesn't seem to accomplish anything after 20...Qa3.

20...Bxc1 21. Qxc1 Qb1 22. Qxb1 Bxb1 23. Nc3 Be4

24. Bb5

24. Nxe4 dxe4 25. Ne1 (25. Nh4 Rg8 26. Nf5 Rxg5, when the idea of stifling Black's rook by 27. g4 Ne7 28. h4 would be met by 28...Nxf5 29. hxg5 Ng3+ 30. Kg2 Nxh1 31. Kxh1 Kg7 and the capture of the g5-pawn) 25...Nd4 26. g3 (intending 27. Ng2; 26. Bd1 and 27. Nc2 would fail to 27...e2+) 26...h6 27. gxh6 Rxh6 28. Ng2 Rf6+ (to coax the knight's interposition and thus preserve the e3-pawn) 29. Nf4 b5, with White's having to disentangle.

24...Ke7

24...Bxf3 25. gxf3 Nb4 (25...d4 26. Ne2; 25...Nd4 26. Be2) 26. Ke2 d4 27. Rb1 Nc6 28. Nd5 h6 (anticipating that the pawn would become a target at h7) 29. g6 fxg6 (29...Rg8 30. Bxc6 bxc6 31. Nf4 b5 32. Rc1) 30. Nxb6 Ke7 31. Bd3, and Black's 24th move has given White's bishop free rein.

25. Ke2

25. Nxe4 dxe4 26. Bxc6 bxc6 27. Nd4 Ra8, and Black seems okay.

25...Ra8 26. Kxe3 Ra3 27. Kd2

Material would be lost by 27. Bxc6 Rxc3+ or 27. Rc1 Bxf3 28. Bxc6 d4+ 29. Kxf3 bxc6.

27...Bxf3 28. gxf3 d4 29. Nd5+

29...Kd6

The attempt to protect the h-pawn with 29...Kf8 30. Nf6 Kg7 would be passive and invite Rh1-e1-e8-g8.

30. Bxc6

30. Nf6 would be met by 30...Ne5 31. Be2 (31. Nxh7 Ra2+ with 32...Ra1+) 31...Ra2+ 32. Ke1 Ra1+ 33. Bd1 Nxf3+.

30...bxc6 31. Nf6 c5

31...Rxf3 would lose a tempo in the pawn race.

32. Nxh7 c4, and Black's counterplay seems sufficient to draw.

C. 6. d5

Here theory gives 6...gxf4 7. e6, but Black has a more appropriate response.

6...dxe5

Since White is bound to generate kingside threats, Black must open the center as a step toward counterplay.

7. fxe5 Bg7 8. e6

Now 8...fxe6 would be suicidal, for example, 9. Qh5+ Kf8 10. Nh3 h6 11. Bd3.

8...O-O

Black will need to rely on his lead in development to compensate for his fragile kingside.

To be examined:

C1. 9. exf7+
C2. 9. Nc3
C3. 9. h4
C4. 9. Bd3
C5. 9. Qh5

C1. 9. exf7+

9...Rxf7 10. Nf3 h6 11. Bd3 c6 12. dxc6 Nxc6 13. O-O Be6 14. Qe2 Nb4

15. Qxe6

15. Bg6 Rf6.

15...Qxd3 16. Qxf7+ Kxf7 17. Ne5+ Kg8 18. Nxd3 Nxd3 19. Nc3 Bd4+ 20. Kh1 Nf2+, with perpetual check.

C2. 9. Nc3

9...fxe6 10. Bxg5 Qd6

The threat of 11...Qe5+ gains control of the c5 square without loss of tempo. Instead, 10...exd5 11. c5 would be unpleasant for Black.

11. Nf3 exd5 12. cxd5 Bg4, and Black's pieces are very active.

C3. 9. h4

9...Qd6 (threatening ...Qg3+), and White's king seems the more vulnerable after 10. Rh3 fxe6 or 10. Qd3 g4.

C4. 9. Bd3

9...h6 10. Qh5 Bxe6 11. Bxg5

11. dxe6 Qxd3.

11...Nxd5

11...Bxb2 12. Qxh6 f5 13. Qg6+ assures White at least a draw, as 13...Bg7 14. Bh6 Rf7 15. dxe6 is crushing.

12. Bxh6

12. cxd5 Qxd5, and Black regains the piece with interest.

12...Nf4

"Simplest."

13. Bh7+

Gaining time over 13. Bxf4 Qxd3.

13...Kxh7 14. Bxf4+ Kg8, and White's in trouble.

C5. 9. Qh5

9...h6 10. h4

10. Bd3 Bxe6 transposes into Variation C4.

10...Qd6 11. hxg5 Qg3+ 12. Kd1

12. Kd2 loses a bishop to 12...Qf4+.

12...fxe6 13. Nf3 exd5 14. Rh4

14. Qg6 Bf5 15. Qh5 Bg4.

14...dxc4

Opens the d-file while preventing 15. Bd3.

15. gxh6 Bf6

White's attack seems to have run out of steam.

Conclusion: The move 5...g5 appears to hold up.