The Solution
1 Bxf5!
The combination which leaps to the over-the-board player's eye is 1 Ra2+ Qxa2 2 Qb4 mate,
but this doesn't work because ...Qxa2 pins White's queen. If White plays the preparatory
1 Kf8 (1 Kg7 is met the same way), then 1...Qb2! prevents Ra2+ and delays mate beyond the
fourth move. Another idea is 1 Bg2, intending 2 Bd5 and then 3 Ra2+, but Black plays 1...b5
and White has to waste time moving the queen. White must move his king off the dangerous
a2-g8 diagonal with gain of tempo, and this is where the bishop on h3 comes into the
scene.
The solution to White's dilemma, if Black doesn't take on f5, then the bishop has entered
play with gain of tempo. Then White's attacking force is too much, for example 1...b5
2 Qc5+ Ka4 3 Bxb1, 1...Qb3 2 Be6! Qxc4 3 Bxc4 and 4 Ra2 mate, or 1...Qb2 2 Qc5+ Ka2
(2...Ka4 3 Rxb2 and 2...Kb3 3 Qb4+ also lead to mate) 3 Qa5+ Kb3 4 Be6 mate. And if Black
does take on f5, the next move is
2 Ke7!
Despite the open position, this careful choice of square leaves Black without a reasonable
check. Black can give up his queen, but this only delays mate by one move. Consequently,
there is nothing better than to stop the threatened mate on a2.
2...Qb1
And now the original idea appears in full force.
3 Ra2+
3...Qxa2
4 Qb4 mate.