To what extent does the war on terrorism represent a break with previous United States foreign and defense policy?
I would say that the war on terrorism does not represent that much of a break with previous US foreign and defense policy. The US has engaged in what were essentially preemptive wars before, but it has not previously fought a long conflict against as amorphous a foe as international terrorism.
One thing that seems different about the war on terrorism is that the US is fighting against something that may be a danger to the country, not something that has actually attacked the United States. Of course, Al Qaeda did attack the United States, but the US is fighting all Muslim terrorists, not just those affiliated with Al Qaeda. There are those who have characterized this as an optional war that was not forced on the US. However, I do not think this is terribly unusual for the United States. During the Cold War, the US fought communism everywhere that it was trying to expand. The US did not wait for a given communist insurgency to attack it. Instead, it fought communists even in places, like Vietnam, that did not seem to be much of a threat to the United States.
What is different this time is how amorphous and elusive the enemy is. The United States has almost always fought wars against countries. It is true that the US has fought, for example, Filipino guerrillas who were not a formal national army and Viet Cong fighters who were part of an insurgency. But the US has never really fought a group (or set of groups) that is not really connected to a given piece of territory and which is not organized as an actual military. The war on terrorism is different in the sense that it is not a fight against a defined group of people. Instead, it is a fight against anyone (and these people can pop up anywhere in the world) who wants to commit terrorist acts. In a sense, this is more like a very extended law enforcement action than like a war of the sort that the US more commonly fights.
Thus, the idea of a preemptive war is not much of a break with the past, but a war against such an undefined and unmilitary group is.