Find the decisive blow (difficulty 1650)

87654321 abcdefgh

White to move

Show solution
87654321 abcdefgh
  1. c6–b5 key move
  2. b8–a7
  3. b5–a5

What makes it

A queen move and a 4-point investment: mate in ten. It calculates 19 plies deep. Rated 1650 — punishing to calculate.