Andy Latto asks the key questions. Is there an en passant capture? Is castling allowed? The rules of chess composition at http://www.wfcc.ch/1999-2012/codex/ esp. article 16, solve this: 1) Castling is permitted unless it can be proved that it is not 2) En passant capture is not permitted unless it can be proved that it is available. Because the last move could have been Kd7-e8, we cannot prove that that the last move was g7-g5 (and, agreed, it was certainly not g6-g5). Therefore we must assume that an en-passant capture is not permitted, and that the last move was by K or R. Therefore castling is not permitted, and so Kf5-e6 is followed by Rd8#. ... but in fact, the assumption is not necessary. We do not even have to rely on the Codex of Composition to resolve the ambiguities of the position inherent in the diagram: If g7-g5 had been played, as stated, h5-g6 followed by Rd8# suffices. G