Removing the guard or defender. This theme is the very essence of chess logic. If Piece A guards. Piece B. Attack the Piece A and you win one or the other. 

360 Brilliant Endgames, click on the picture to view the endgames. 
ENDGAME 1
ENDGAME 5
PROBLEM 7
ENDGAME 9
ENDGAME 2
PROBLEM 4
ENDGAME 6
ENDGAME 10