If the rook weren’t on f2 then White could immediately give mate with the knight. Unfortunately, moving the rook out of the way gives Black time to play 1. … Bd4, and now White’s plan doesn’t work — if the knight checks the king then the bishop can simply capture it.
The answer is 1. Rf6! This stops the bishop from reaching d4, and Black has no way to stop 2. Nf2#.