It is really quite simple and makes perfect sense. I'm not sure what is confusing you. When you attack them over there, destroying their resources and killing their mindless followers, they are unable to plan, organize, and carry out attacks in the U.S. When have to engage them over there because it allows us to destroy their training camps, demolish their "schools" where they indoctrinate the naive, and establish intelligence relationships. You should do some reading on the Afghan campaign in the early days after 9/11. We had some major victories against al Qaeda. Here's an example of what can be accomplished with strong intelligence: