There are many varying philosophies of defense. Basically, two dominate contemporary thought. The first I refer to as "The Fortress." This defense acts on one premise alone: Stop the Offense from scoring. The bend but don't break defense is a good example of this philosophy. This is not the approach preferred by this author. Fortresses are designed to weather the storm and this strategy is a defensive approach to Defense (pardon the play on words).
I prefer to utilize the "Divide and Conquer" strategy instead. I favor the offensive approach to Defense. The Defense needs to attack the Offense. The Divide and Conquer defense places the defense's responsibilities on a higher plane. Goals for this defense are to deny every yard, every pass, every play.
In this strategy it is the responsibility of the defense to return the ball to the offense and to keep the opposing offense from dictating the pace of the game. Defense should emphasize creation of turnovers and forcing the offense into long yardage situations. An offense kept under constant pressure is more likely to make mistakes. When a mistake is made, the defense must be skilled at taking advantage of the mistake. It serves little purpose to make great efforts to cause a fumble by stripping the ball if the defenders are not skilled (through practice drills) at recovering fumbles.