A challenge to mercantilists

A challenge from Don Boudreaux at the Cafe Hayek blog. Boudreaux issues the following challenge to any protectionist/mercantilist/economic-nationalist who might wish to take it on:

Identify one plausible economic problem caused by free trade that is unique to trade and commerce that spans political borders. Just one. That is, identify a problem with free trade that arises when people are free to buy and sell internationally but that does not arise when people are free to buy and sell intranationally.

In other words, what economic problems can we avoid by restricting international trade that we don't have to deal with in internal trade?  I've got to admit I've yet to think of anything even remotely plausible as an answer to this challenge.