Acting White House chief of staff Mick Mulvaney said that the president’s move to impose tariffs on Mexico stemmed from “extraordinary circumstances” at the border, where Central Americans who have traversed Mexico are flooding into the United States.

Speaking Sunday on NBC’s “Meet the Press,” Mulvaney said that the U.S. is facing a crisis of proportions never before seen, and so Trump imposed a 5% tariff on goods from Mexico.