Originally posted by David113
Mate in 9 moves.
White needs to promote a pawn to add a third attacker. Black has several ways of playing out his pawn tempos. White must choose his promotion carefully to provide a timely unpin once the pawn tempos run out.
1...a5 2.d5 a4 3.d6 a3 4.d7 a2 5.d8=N f6 6.Nb7 N~ 7.Nc5+ Ng2 8.Ne4 N~ 9.Ng3#
1...a6 2.d5 a5 3.d6 a4 4.d7 a3 5.d8=R a2 6.Rd4 f6 7.Re4 Ne1 8.Rxe1 a1=Q [8...Kg2 9.Rf4#] 9.Ra4#
1...a5 2.d5 Nf4 3.d6+ Ng2 4.d7 a4 5.d8=B a3 6.Ba5 a2 7.Bxc3 f6 8.Be1 Bxf2 9.Bxf2#
1...a6 2.d5 Nf4 3.d6+ Ng2 4.d7 a5 5.d8=Q a4 6.Qd5 a3 7.Qf3 a2 8.Qxh3 f6 9.Bxg2#
AUW. Excellent composition - it is not easy to do this in a long directmate.