Explanation

Protecting a square is insufficient, if the protector can be captured.

The following diagram illustrates the most common ways to remove a defender of checkmate:

White is attacking g7 with the battery on the g-file. Capturing with the queen on g7 would be checkmate if the square weren’t defended by the black knight on e6. But Black's defense is not solid, and White can remove the defender in several ways to win material or force checkmate:

• White can capture the defender with 1.Bxe6. If Black recaptures, White can checkmate on the now-unprotected square g7: 1...Bxe6 2.Qxg7#. Black can avoid checkmate with 1...g6, but then White has won the bishop.
• White can attack the defender with 1.f5. If Black runs away with the knight, g7 is unprotected once again: 1...Nf8 2.Qxg7#. Here Black can also avoid checkmate with 1...g6, but then White captures the knight with 2.fxe6.
• The black knight is also protecting the square d8 against a back-rank mate by the white rook on d1. Guarding two checkmate squares, the knight is overloaded and can be deflected with 1.Rd8+! Nxd8 2.Qxg7#. This is even better than capturing or attacking the defender in this case, because checkmate is forced.
• The knight can also be deflected in the other direction: 1.Qxg7+! Nxg7 2.Rd8#.

Examples

Black could deliver checkmate with 1...Bh3+?, if it weren't for the knight on f4 protecting h3: 2.Nxh3.
Black can win by capturing the defender with 1...Nxf4.
If White recaptures with 2.gxf4, Black can checkmate on the now unprotected h3 square with 2...Bh3#.
White is unable to prevent checkmate with any other moves. After 2.g4 Black can just play 2...Bxg4 and renew the checkmate threat, which cannot be prevented.

Sometimes capturing the defender can be a sacrifice, as in the following example:

Black has a weak back-rank, which is defended only by the rook on c8, while White has two pieces which can move there: The queen on c2 and the rook on e1.
White can win by capturing the defender with 1.Qxc8+!.
If Black recaptures with 1...Bxc8, White can checkmate with 2.Re8#.
If Black blocks the check with 1...Rd8 White can force back-rank mate anyway with 2.Re8+ Rxe8 3.Qxe8# (1...Qd8 works in the same way).

This is a common way to force back-rank mate.

How to Spot an Opportunity to Capture the Defender

When you are threatening checkmate, but the checkmate square is protected, always think about ways to remove the defence.

Related combinations

There are various ways to remove the defence. In addition to capturing the defender, other ways to remove the defence include attacking the defender, and deflection.