1.Rg3-d3 (threatening Qe5-g3#)

Of all the possible Black replies, only moving the Pawn prevents Qe5-g3 from being mate (because it opens e2 as an escape square for the King):

1. 
   1. ... e2-e1=Q
   To which White replies:
   2. Rd3-f3#
   
2. 
   1. ... e2-e1=N
   To which White replies:
   2. Rd3-d2#