(1) P-f6. The only move to protect the mate on g7 is P-g6. It
will then be White's desire to enter with his Queen on h6 again
threatening the checkmate on g7. He will, therefore, play either
Q-f4 or Q-g5. Black has no other way to defend himself against
White's threat than by K-h8 followed by R-g8. But after K-h8; Q-
h6, R-g8 White plays R-f3 and Black is unable to prevent White
from sacrificing his Queen on h7 and mating by R-h3.

If Black's Queen were standing on c5 instead of b5 he could avert
the mate by moving the Rook f8 and playing the Queen to f8
thereby protecting g7.

+---------------------------------------+
8 | #R | | | | | #R | #K | |
|---------------------------------------|
7 | #P | #P | | | | #P | #P | #P |
|---------------------------------------|
6 | | | #Kt| | | | | |
|---------------------------------------|
5 | | #Q | | #P | ^P | ^P | | |
|---------------------------------------|
4 | | | #P | | | | | |
|---------------------------------------|
3 | | | | | | | ^Q | |
|---------------------------------------|
2 | ^P | ^P | | ^B | | | ^P | ^P |
|---------------------------------------|
1 | ^R | | | | | ^R | | ^K |
+---------------------------------------+
a b c d e f g h