I've got a foot in both of these "camps". Though I tend to wonder, why couldn't isolationism work? After all we tended isolationist from 1848 - 1898 and then from 1899 - 1940. Of course we would need to boost some technology development a bit.
Steyn is right, with so many people not believing in value such as individual right to self-determination and such at home, it ispointless to pursue it abroad.
With more than a million Muslims here, many of whom would like to forcibly impose Islam on us, and 15-or-so million Hispanic immigrants, it's a bit late for isolationism. We imported trouble and now we have to fight the root causes of it.
If we had truly remained isolationist and had retreated to our side of the planet after World War II, Communism would have taken over much of the world. We would be entirely outnumbered and surrounded by Communist aggressor nations, people who were as hypnotized by their ideology as the Islamists are by theirs, and almost as determined to install it here.
My big problem with pursuing "self-determination" abroad is that, witness Iraq, most Muslims will self-determine immediately to adopt sharia and enslave themselves again. It was a bad move to let them enshrine sharia in their constitution, although the only thing I can say in defense of Bush et al. is that it has taken everybody awhile to learn and understand what Islam really is (a tyrannical religious-political system) and at that time, most folks hadn't really grasped it.
Theoretically, it is not legally possible or permissible to sell yourself into slavery. But many of the people we liberate or people who live in the free world seem ready to do just that.