It’s time to improve your tactical skills for the New Year.

White to play: what move would you suggest?

Last week I left you with a rather pretty Christmas Tree.

How can White mate in two moves? It looks impossible at first sight, but if you do some retroanalysis you can find the answer.

It’s easy to mate in two if you can make an en passant capture, but can you prove that this is legal?

Black’s last move couldn’t have been with the king: if he came from d8 or f8 he’d have been in an illegal double check, and he can’t have moved from d7 or f7 because there’s no way the white pawn on e6 could have reached there the previous move.

So Black has just moved a pawn. The pawns on b7 and h7 clearly haven’t moved, and the pawn on e4 could not have moved there last move.

Now we know that Black’s last move must have been either d5 or f5. If either pawn had moved one square White would have been in check with Black to play, so Black’s last move must have been either d7-d5 or f7-f5. White can now capture en passant and mate next move.

White has made 10 pawn captures (count them!) to reach this position. Black has six units on the board, so all the missing black units were captured by white pawns. Finally, we understand that Black’s last move couldn’t have been d7-d5 because Black’s light-squared bishop didn’t die at home, but was captured by a white pawn. Therefore we can demonstrate that Black’s last move could only have been f7-f5 so White can play g5xf6 (en passant) followed by f6-f7#.

Elementary, my dear Watson, or should I say Dawson?

Yes, this problem was composed by the great Thomas Rayner Dawson (see here, here and here).