1. Rd1 denies a vital flight square to the black king. Now:

1. … Kc3 2. f7#
1. … Kc5 2. Qg1#
1. … Ke5 2. d4#
1. … Ke3 2. Qg1#

Even with the advantage of a full army, this is White’s only sure way to mate in two moves.