These columns favor generous immigration and asylum for refugees. But when migration becomes a political weapon to foment border chaos, leaders have no choice other than to step in and protect national security. Exhibit A are the 4,000 or so Central Americans moving on foot through Mexico to the U.S.

Waves of humanity marching in lock step don’t materialize spontaneously and neither has this “caravan.” This march is organized and not necessarily for the benefit of the migrants. Mr. Trump has good reason to turn it back.

...