Originally posted by AmauroteYou don't want to move the bishop off the a2-h7 diagonal to keep b1 covered, also the black rook can't stop the threat of mate by 1... Ra6 if the bishop goes to d3. So after 1.Bd3 checkmate is threatenned on a4 and black has 1. ... Ka2 2. Bc4+ which drops the rook, 1. ... Re1 2. Kc2 is good for white as is 1. ... Be3+ 2. Kc2 Rc5+ 3. Bc4.
It looks like White after Bd5 setting up a mating pattern, but I dread to think what Black is doing with his rook in the two half-moves of the set-up...