Chess Online

William L. Barclay, Deutsche Schachzeitung 1966

But what happens if the black blocks the check with 1... Qe4? The queen can't be captured because it's supported by the bishop on b1, so the best moves appear to be 2. Nf3+ Kc3 3. Bd2# or 2. Bxe6 with 3. Qd5#/Bg7#/Nb3# depending on what move black plays, but either way these are mate in 3 and not 2.

I solved this after quite a long time; I might have done it faster if I had managed to convince myself earlier that the key move might be a check.

Even then, the key move is probably the last square I would have thought to place the knight in an actual game, because of the fact that it opens up two extra flight squares for Black - but it's the only square that works. I also thought the answer was going to be 1. Nd2+, but after 1...Qe4, neither of the two knight moves 2. Nb3 or 2. Nf3 achieves mate. That was when I realised the knight had to go to c5 so that it could mate by 2. Nxa4#.