Historically speaking, Westphalian sovereignty meant that there was no such thing as an "international criminal court", nor "war crimes". An ICC party such as France hypothetically arresting, for example, Netanyahu, for things that he did in Israel, would amount to a substantial erosion of Israel's sovereignty in the Westphalian sense. Under the Westphalian system, Israel's prince would have sole jurisdiction in such cases.

Of course, that doesn't necessarily tell us anything about whether it's good or bad. Eroding Westphalian sovereignty in such a sense is the whole point of the ICC, the EU, and arguably even the UN (though, of these three, only the ICC would have the particular result described in my previous paragraph). But it's worth pointing out that it's a major difference from centuries of historical precedents, not American exceptionalism.