"Fortress America" became impractical a long time ago.
A lot of staunch conservatives and libertarians feel that way, but it ain't 1940 any more, and the era when we could fight a World War and lose only six Americans on our own soil is long gone.
Until somebody comes up with a World Policeman that does a better job than we've been doing for two generations, America's still stuck with the job. Unless we abandon it.
And the first person who brings up the United Nations as a "peacekeeping force" will get hit with cream pies until the mound of pastry is forty feet deep.