Find the decisive blow (difficulty 1125)

87654321 abcdefgh

White to move

Show solution
87654321 abcdefgh
  1. b2–a1 key move
  2. f8–b4
  3. c2–b4

What makes it

Best defence still loses — mate in five. No check, no capture: just a quiet move, and the defence caves in. 9 half-moves of forced play.