Mate in 1140:

for which side?

Did you really think for almost 10 years about this?

I am surprised nobody posted this LEGIT one in almost 10 years. Mate in 271!

...

Ten years ago, there were several posts all on the same subject.

Here's a link to one site that discusses some of the longest possible forced mates if we disregard the 50 and 75 move draw rules:

https://chess.stackexchange.com/questions/13178/what-is-the-longest-forced-win-with-3-to-6-pieces-ignoring-50-75-move-rules#:~:text=Longest%20known%20checkmate%20is%20in,can%20see%20the%20game%20here.&text=is%20not%20useful-,Show%20activity%20on%20this%20post.,is%20a%20mate%20in%20549.

And here's a link to a thread on the longest checkmate known so far that incorporates the 50 and 75 move draw rules:

https://chess.stackexchange.com/questions/21723/what-is-the-longest-proven-forced-win-including-50-75-move-rules?noredirect=1&lq=1

Mate in 1140:

for which side?

Did you really think for almost 10 years about this?

I am surprised nobody posted this LEGIT one in almost 10 years. Mate in 271!

If Black ever moves the Knight, it is game over in a few moves with Bxf3. The White King cannot be allowed in a6. The Black King must toggle between a8 and b7 or c8 and b7. It cannot triangulate and so it cannot lose a tempo. If it were Black to move, he wins with 1...d3. It is White to move though, and after 1 Bishop move and pawn move by Black, he must lose a tempo.

1.Bb1 h4 2.Ka4 (2.Ba2?? d3-+) Ka8 3.Ka3 Kb7 4.Ka2 Ka8 5.Ka1 Kb7 6.Ba2 Ka8 7.Kb1 Kb7 8.Kc1 Ka8 9.Kd1 Kb7 10.Ke1 Ka8 11.Kf1 Kb7 12.Kf2 Ka8 13.Ke1 Kb7 14.Kd1 Ka8 15.Kc1 Kb7 16.Kb1 Ka8 17.Ka1 Kb7 18.Bb1 Ka8 19.Ka2 Kb7 20.Ka3 Ka8 21.Ka4 Kb7 22.Ka5.

Now Black must move a pawn, either f- or h-.

Repeat process until Black is forced to either

A) Move the d-pawn with Bishop on b1

B) Move the Knight, which loses to Bxf3+

C) Move the King, which loses to Ka6.

Longest Black can go is 271 moves.