Find the decisive blow (difficulty 1530)

87654321 abcdefgh

White to move

Show solution
87654321 abcdefgh
  1. d7–a4 key move
  2. b3–a5
  3. a4–a5

What makes it

Mate is forced in 15. A fork: two or three pieces fall under attack together. 29 half-moves of forced play.