if a real full scale convetional war broke out between the US and someone with a decent army, most of the US navy would be destroyed quite fast. They're all big lumbering surface ships. Just look what happened hte Brits in the falklands. Many ships lost to only a small number of exocets.
Their submarines would probably be fine though.
EDIT: The first 2 pages of this thread, including this post, are merged from other threads.