How many moves can you calculate?
Logic
Level
2
White to play. Assuming that white and black play optimally, what is the minimum number of moves for white to checkmate the black king?
2
4
1
5
0
3
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1 solution
If White promotes a pawn immediately by 1. e8=Q or 1. g8=Q, then Black replies 1...Bb8, and White cannot take the bishop because of stalemate, so there is no mate in two. The correct plan is to arrange for the rook to be on the other side of the queen, so that ...Bb8 is no longer an effective defense. However, 1. Rg8? (1 Rf8? is also met by 1...Bd6) fails to 1...Bd6 2 e8=Q Bf8, blocking the back rank.
The rook must go to the very end, so as to give White a choice of promotions. White doesn't threaten anything, because 2. e8Q and 2. g8=Q would be met by ...Bd8, but Black's bishop cannot stay on c7. After 1...Bd6 White plays 2. g8=Q, intending Qa8 mate, while if the bishop moves to any other square on the b8-h2 diagonal, White forces mate by 2 e8=Q.