In the first position (and, of course, the second too) Black's king is completely surrounded, but clearing away the clutter and getting a piece through to deliver mate is not easy. 1.Ne4+ Kxe2 2.Ng3+ Kxf3 fails to mate within the specified time, as does 1.Bxe3+ Kxe3 2.Re4+ Kd2.
The solution is surprising: the only move to force mate in four is 1.b8=N!! when Black is remarkably helpless against the threat of 2.Nxd7 3.Nc5 and 4.N5e4 mate. 1...d5 is met by 2.Nc6 dxc4 3.Ne4+ Kxe2 4.Nd4 mate.
So what is the difference in the second position? Why doesn't the analogous solution work? The answer is delightfully hidden: after 1.c8=N Black plays 1...Rxh1! 2.Nxe7 g1=B! when 3.Nd5 leaves Black stalemated.
The solution this time is 1.Nf4+ Kxf2 2.Nxh3+ and now 2...Kxg3 is met by 3.Nf5+ Kxh3 4.Bg4 mate, while 2...Ke2 allows 3.c8=Q! followed by Qa6 mate - both mates impossible in the first position.Reuse content