Originally posted by Mephisto2
Not very difficult problem without words (i.e. figure out what the problem is and its solution).
[fen]8/6P1/5QP1/5PPP/4BpPk/5P1p/4R2K/6BR w - - 0 1[/fen]
White's b- through e- pawns all captured their way to the Queenside, consuming all 13 missing Black units in the process.
bPh is behind wPh, which never left the file. White took the g-pawn, so this pawn was originally on h7 and required two captures to get around wPh.
The collision of the a-pawns means that either Black must make a third capture to get around it, or must have another piece kill wPa. Either way, this constitutes Black's third and final capture.
Therefore, the last move was not ...e5xf4. There are no captures left for it. Black has no legal last move.
The last moves were -1.g2-g4 Kg4-h4 -2.f2-f3+. There's no other way to give Black a legal retraction.
Therefore, it is Black's move, and he has no choice but to play