Find the decisive blow (difficulty 945)

87654321 abcdefgh

White to move

Show solution
87654321 abcdefgh
  1. a3–b5 key move
  2. g7–a1
  3. b1–a1

What makes it

Mate is forced in three. No check, no capture: just a quiet move, and the defence caves in. Difficulty 945: a warm-up.