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Protecting a piece is insufficient, if the protector can be captured.
The white rook on c1 is attacking the knight on c7, which is protected by the black queen on d8.
White can trade queens with Qxd8, capturing the defender of the knight, which is now unprotected.
After Black recaptures with one of the rooks, White can win the knight with Rxc7.
Black's bishop on d6 is attacking White's bishop on f4, which is protected by the knight on e2.
Black can capture the defender with Bxe2, leaving White's bishop unprotected.
If White recaptures, Black can win the bishop with Bxf4.
Note that White cannot play the intermediate move Bxd6 to safe the bishop, as Bxe2 is attacking White's queen on d1.
Sometimes capturing the defender can be a temporary sacrifice, if the material won subsequently is worth the investment, as in the following example:
White's queen on d2 is attacking Black's rook on d4, which is protected by the bishop on g7.
White can temporarily sacrifice the exchange with Rxg7+, capturing the defender of the rook, which is now unprotected.
After Black recaptures with Kxg7, White can win the rook with Qxd4+.
The investment was a good one, in the end White has won the bishop on g7, and has a winning material advantage.
When you are attacking a protected piece, always think about ways to remove the defence.
There are various ways to remove the defence, of which capturing the defender is the easiest. Other ways to remove the defence include deflection, attacking the defender, and interference.
It is also possible to capture the defender of a square, for example of a square where a checkmate, or a fork is possible. You can learn these combinations in the following levels: