Originally posted by Mephisto2
[fen]r3k3/ppp5/2p2P2/3PpK2/8/NP1R4/p1P3P1/1b3N1R w q - 0 1[/fen]
White plays and mates black in two moves. Not as easy as it looks at first sight!
White is missing six units; Black needs to make six pawn captures. All missing White units were captured by Black pawns.
White is missing a dark-square Bishop. This means Pa2 did not come from f7, for Black would have made all his captures on light squares. The only alternative is that the pawn on e5 is not the original e-pawn.
Black's last move was not ...e7-e5
If Black's last move was ...d7xc6, then Bc8 died at home and Bb1 is promoted. However, a Bishop promoted on h1 could never have left the square, and Black has no spare pawn captures to reach f1 (the e- and f- pawns are already spoken for). Promoting on d1 or b1 needs even more P captures than f1, so these are also impossible.
Black's last move was not ...d7xc6
Since Black's Pe5 was the f-pawn, it has no last move. The only remaining possibility is that Black moved the King or the Rook on his last move - therefore, he can't castle
. White mates by 1.Ke6.