In this year of Brexit and Donald Trump and ethno-nationalism rising across Europe, it is time to take stock of what we have learned.

Even in an era when globalization is thought to be an inexorable force, national sovereignty still matters. It was national political decisions that created the European Union and may end up dismantling it. Political leaders reached the trade treaties that allow goods and services and capital to flow more freely. Many countries welcome these flows yet seek control over the movement of people...