1. Nxd1
(The Knight moved from b2, capturing the Knight on d1.)
Now White threatens Qf2#, and the only move that prevents that is:
1. ... Qe2 to which White replies:
2. Rg1#
Now let's look at the more difficult solution to our original problem, in fact, let's remove the easier solution by removing the Knight from b2, so that the above solution is no longer possible:
White to Move, Mate in Two:
