Figure[Black to move]

Again you see Black poised to mate, this time with his queen and knight aimed at g2. But g2 has protection. The immediate thought of the interference move Re2 is inadequate for a couple of reasons. The first, of course, is that g2 is guarded not only by White’s queen but by his knight on e1. But even if the White knight were out of the picture, Re2 wouldn’t work because of the terrible threat White has at the other end of the board. When an enemy knight is deep in your territory, fear of getting forked should be ever present in your mind; here the knight at c6 is one move from forking Black’s king and queen. Black's rook currently prevents this by guarding e7, but if that rook were to be captured, the fork would be effective. So if Black plays 1. …Re2, play proceeds 2. QxR, BxQ; 3. Ne7+, Kf7; 4. NxQ, KxN (or NxN). White has extinguished the mate threat, won back his queen, and won the exchange to boot.

Not that you should give up. When you see a key point in the enemy camp (like g2) guarded along a long, open line (like the second rank), think hard about ways to interrupt the protection. Black has another piece besides the rook with which he can experiment: his bishop. Consider Bc2. It again creates an immediate threat of mate on g2, so White has to do something about it:

(a) If White plays RxB, Black mates on the spot with RxNe1#.

(b) NxB won’t help, as it leaves the White queen’s path to g2 blocked; Black mates immediately with Qxg2#.

(c) White’s best reply is QxB; then if Black plays QxQ, White has NxQ and the mate threat is over. But in reply to White’s QxB Black still has a different reply that is decisive: RxN+. (Consider every check you can give in these circumstances.) White is forced to play RxR to stop the threat on his back rank. This leaves White's queen loose (his rook was overworked, protecting both the queen and e1), so Black is free to play QxQ. Now Black still threatens mate with Qxg2, so White’s only move is Rg1, protecting the g2 square but smothering his king. Black has more than an advantage; he has a forced mate. (After White’s Rg1, it goes 1. …f4-f3; 2. g2xf3, Nxf3; and now White has nothing to do but throw useless checks at Black’s king while waiting for Black to finally play Qxh2#.)