The US has two borders. One with Canada, one with Mexico. Neither of these countries are likely to invade.
The US has the largest naval force in the world. I believe the Coastguard is also the 13th largest. Again, nothing to fear.
Of the countries that are "enemies", most would quite like to improve relations, for instance Cuba and Iran. Iran in particular has extended many an olive branch over the decades.
The US keeps these countries around as notional enemies, in part because of political incompetence, and in part because of a lack of incentive to fix things. Who wants to be the President who made friends with an "Islamic Republic"?
Either way, it's hard to see what particular danger the US is in.
Compare this to any of the countries I mentioned, which are much less defended, and have active disputes with neighbouring countries.