If I'm allowed to put on my Historian hat, I would say that the reason for all this military engagement stems from a sense of "mission" that is quintessentially American.
One of the most influential books I read in college was Manifest Destiny and Mission in American History
. Although it focuses on events in the 19th century, Frederick Merk really knew well the American mindset that pervades all of American history, specifically in terms of that sense of "mission" that most Americans have. Basically, we consider ourselves destined to be that "shining city on a hill" that Ronald Reagan liked to talk about. We're crusaders, bent on saving the world from its own destruction.
At the same time, however, we like our politics, and we like our money, which means that both play a role in military matters. Currently, the big dollar item globally is oil, and it comes as no surprise that America's "interests" now focus on the oil-rich middle East. First with Kuwait, then Iraq, and now Iran (all while staying friends with Saudi Arabia, and not to mention the latest endeavour in Libya), the United States has worked to keep the oil flowing, and not even to itself, necessarily. Iran really poses no military threat to the United States, even if it did
have nukes. But, well, we've got to have our mission, ostensibly to protect the world from a nuclear Iran, when in all likelihood, it's probably all simply about oil.