One big problem with nationalism is that it is a leading cause of mass
murder. Fascism and Nazism were, of course, extreme forms of nationalism and the
mass murders Nazi and fascist regimes committed were justified on the grounds
that they were necessary to advance the interests of racially or ethnically
defined peoples. Obviously, most nationalist governments do not commit mass
murder on that scale. This is one reason why nationalism is not quite as
pernicious as socialism. Nearly all full-blown socialist regimes that have lasted for any length of time
have engaged in mass murder; "only" a substantial minority of nationalist
regimes have done the same.
But many non-mass murdering nationalist regimes still use nationalism as a
justification for protectionism, discrimination against minority groups,
suppression of dissent, and the like. Nor are these abuses simply the result of
misinterpretations of nationalism by unscrupulous rulers. To the contrary, if
you genuinely believe that we have special obligations to members of your ethnic
or national group that sometimes trump universal principles, consistency
requires that you be willing to sacrifice the rights of other groups to benefit
your own, at least sometimes.
Hoppe's position that keeping illegals off public property because of their supposed "invasiveness" could easily be extended to other matters, aside from free trade. Gun laws, drug laws, prostitution laws, drinking laws, smoking laws, laws against prayer--all of these things could be defended on the basis that many tax-paying property owners would not want such behavior on their own private property. Such examples are hardly without a real-world basis. Large numbers of Americans would not allow guests in their homes if those guests had machineguns or crack cocaine in their possession. The principle of the freedom to exclude and set conditions for entry onto private property simply cannot be extended to the socialized public sphere, or else all sorts of unlibertarian, illiberal policies could be as easily justified as border controls. In other words, just because an individual--or many individuals--would not want act X to occur on their property does not mean that, according to libertarian law, it can be prohibited as a general principle, even on so-called "public property."