The march from Honduras echoes the 1980 Mariel boatlift.

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.

Not since the 1980 Mariel boatlift from Cuba has there been a similar attempt to overwhelm U.S. immigration law on the pretext of celebrating American freedom. Thousands of Cubans made their way to Florida when Fidel Castro temporarily lifted his Havana Curtain, and American boats of every shape and size sailed into the Caribbean to collect them. But the sheer magnitude made it impossible to process the newcomers in an orderly fashion. Castro saw to it that criminals and the mentally ill also climbed aboard the boats. No one doubted the Cuban hunger to escape, but the unintended consequences of the mayhem were costly. A Mariel replay now seems to be coming from Honduras. Though the details are murky, we do know that former Honduran congressman Bartolo Fuentes of the left-wing Libre Party has admitted to organizing this caravan.

Mr. Zelaya remains active in politics. While center-right President Juan Orlando Hernández has encouraged Hondurans on the journey to return home and even has offered them assistance, Mr. Zelaya is egging them on.

More From The Wall Street Journal (subscription required):