1. Nc4-a5 (Threatens Qb5xb7#)
There are two ways Black can stop this threat (but they both lead to mate in another way):

1. 
   1. ... a6xb5
   To which White replies:
   2. Na5-c4# (simply returning the Knight to where it started! This uncovers a check from the Rook, and guards the King's would-be escape square: b6.)
   
2. 
   1. ... b7-b6
   To which White replies:
   2. Qb5xa6# (now that the b7 Pawn no longer defends a6.)