http://www.chess.com/forum/view/more-puzzles/mate-in-292-movesblathys-monster?page=1
Forced mates
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Those were all forced?!
The one from the original post was forced.
The longest forced win anybody has ever seen was discovered by a computer and is mentioned in that thread (it takes white 517 moves to convert to a won subendgame against best defense).
The longest forced win discovered by a human is I believe just over 300, but the one listed in that thread is "only" 292 moves. However, both these come from positions that cannot be legally reached in ordinary play.
The longest forced win discovered by a human that results from a legal position is also listed in that thread (in my post) and is a mate in 217.
What's the longest forced mate you've ever seen?