The world has many island nations, and sometimes the United States counts itself among them. We have water on either side of us, and though we share our big island with Canada and Mexico, neither poses any threat. America is immune to invasion, a castle surrounded by the safest of moats.

This wasn’t always so, and it isn’t really true today, unless we forget about Hawaii and our Pacific and Caribbean territories, most of which would be easy prey for other states if they weren’t under our sovereignty. In