White can only mate on two squares e7 and h8, but if 1.Qa3 black has 1...c5 or 2...O-O-O. If 1.Qa1 then 1...O-O-O. At least that's what I thought, but after 1.Qa1 black can't castle because he made the last move and it must have been with either the King or Rook, so castling is not legal.
