The US ARMY could fill up half the world but they still cant prevent wars that take place between other countries- at some point those wars will oblige our involvement and at that point there's no telling how far it might escalate. Currently there are quite a few of these wars taking place- the more the US spreads itself thin to oversee other countries affairs the less capable it is of covering all its angles - to think it has all those bases covered even in peace time is naive.
Dont front the US military is the best- but they aint infallible.
The USS COLE didnt get blasted cuz somebody just got lucky.