So far the United States has been protected by circumstance and technology.

One of your biggest defenders has been "General Ocean" that has protected your mainland from almost any credible foe, bolstered by economic and political warfare waged against anybody that might threaten that- e.g. Cuba.

The days when any imaginable enemy might send a flotilla against a blue water target are long gone, unless the US decides to ;-) and even then the cost would savage the economy and create voter discomfort, with predictable result.

MAD is also over. Nations possessing WMD are no longer inclined to threaten each other with total destruction.

In 2007 the means of murdering millions can be carried in a small vial, so defence has to involve removal of motivation for the sorts of people with the resources to create and use such weapons. The Iraq war may have all the profile but behind the scenes there is *heaps* of work going on trying to demotivate hatred against the Great Satan. That's where international co-operation carries real value IMHO.