Attack defender of mate (2 moves)
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Protecting a square is insufficient, if the protector can be forced away with an attack, leaving the square unprotected.
In this level all puzzles have two moves. The defender that you attack will always be the king itself (otherwise the mate would not be forced, or the puzzle would require more moves).
Rxh7+ would be mate, but the king protects h7.
White can win by attacking the defender with g5+.
The king is forced to h5, from where it no longer defends h7.
Rh7# is now checkmate.
Sometimes attacking the defender can guard additional escape squares, for example:
White can win with Nf6+.
The knight cannot be captured because the g-pawn is pinned by the queen.
Black's king is forced to h8, where it no longer defends f8.
Rxf8# is now checkmate, because the knight also guards the escape square h7.
When you are threatening checkmate, but the checkmate square is protected, always think about ways to remove the defence.
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.