Libertarian advocates of “open borders” sometimes maintain that there should be no difference between an American traveling across a state border and a foreigner traveling across the U.S. border. But if this is what characterizes “open borders,” then that in and of itself is reason enough to reject “open borders” because, carried to its logical conclusion, “open borders” would then mean that if armed soldiers from any country in the world landed on America’s East or West Coasts, or massed at the Canadian or Mexican borders, and said that they were tourists coming to the United States to do some sightseeing, they should not be refused entry. And not only that, they could not be refused entry without violating their freedom to move, travel, migrate, or seek refuge. And if that weren’t ludicrous enough, foreign soldiers should be invited and welcomed just like everyone else in the world. If this conclusion is wrong, and not what certain left-libertarians envision as possibly happening with “open borders,” then perhaps they should use a different analogy or explain their position a little better.. . .
I have not argued against admitting Syrian refugees to the United States. I have not argued that immigrants take American jobs. I have not argued against admitting Muslims to the United States. I have not argued that immigrants commit crimes disproportionate to their numbers. I have not argued against immigrants. I have not argued that some refugees might potentially be terrorists. I have not argued against lawful immigration. I have not argued that some immigrants don’t want to assimilate. I have not argued against unrestricted immigration. I have not argued that some immigrants don’t want to learn English. I have not even argued against “open borders.”
I have merely pointed out some differences that libertarians can and should be recognizing—even if they are sympathetic to “open borders.” (link)