For those who, contra Robert Higgs, recognize that some libertarians are for the war and some are agin', the Volokh Conspiracy's Ilya Somin offers a bunch of reasons for the division, connected to the Milton Friedman/Rose Friedman divide re: Iraq. And one of those reasons is related to immigration:
[A]necdotal evidence suggests that immigrant libertarians are more likely to be pro-Iraq War than native-born ones. So too with Jewish libertarians (who, even if native-born, may have a strong consciousness of their people's oppression by governments outside the US) as opposed to gentile ones, though Milton Friedman is one of many exceptions to the pattern. If you are highly focused on the evils of oppressive regimes and political movements outside the US, you might be more willing to countenance the use of American military power to destroy or contain them than if you have regarded the US government itself as the main threat to your freedom.