Jay: “I advocate open borders.”
Lots of people Left and Right: “What, you don’t believe in national sovereignty?”
Jay: “Well, no, but even if I did, invoking national sovereignty doesn’t resolve the issue.”

Many people think something like the following argument is sound:
1. States have a right to national sovereignty.
2. National sovereignty includes a right to determine who may pass borders.
3. Therefore, national sovereignty precludes open borders.

In this argument, premise 2 does all the work. Most laypeople will just present premise 1 and immediately jump to the conclusion, 3: “Nations have a right to national sovereignty; therefore, they have a right to close or restrict their borders to immigrants.”

However, the problem with this argument, which proponents rarely notice, is that it doesn’t specify why national sovereignty includes this right to restrict freedom but not others. The restrictionist’s argument can be parodied as follows:

Jay: “I advocate free speech, freedom of lifestyle, sexual freedom, free trade, pharmaceutical freedom, and freedom of conscience.”
Illiberal respondent: “What, you don’t believe in national sovereignty?”
Jay: “Well, again, no, I don’t, but even if I did, invoking national sovereignty doesn’t resolve the issue.”

As this dialogue illustrates, it would seem to be a non-starter, or at least not very illuminating, to argue against other liberal freedoms on the grounds that nations or states enjoy national sovereignty. After all, the liberal could just say, “I believe in national sovereignty, but nations have sovereignty over only a limited range of issues. They do not have legitimacy or authority to eliminate free speech, sexual freedom, and so on. The dispute between you (the illiberal) and me isn’t over whether nations have sovereignty, but over what they have sovereignty. So let’s hear your real argument. Please stop pounding the table.”

The defender of open borders can say the same thing. “Sure, nations have sovereignty, within certain limits set by justice. I presume you agree. So, now let’s move on to the actual dispute, which is over whether people have a right to emigrate/immigrate and to what degree nations may restrict that. Please stop invoking sovereignty as if you were making an independent argument rather than just begging the question.”