Find the decisive blow (difficulty 1065)

87654321 abcdefgh

White to move

Show solution
87654321 abcdefgh
  1. e8–c8 key move
  2. a6–a5
  3. b2–b4

What makes it

The win is mate in four, paid for with a 1-point sacrifice. A fork: two or three pieces fall under attack together. 7 half-moves of forced play.