A caravan of migrants, nearly all of them Hondurans, is making its way north through Guatemala toward Mexico and the United States. It is the latest, and certainly the largest, iteration of a phenomenon that has occurred from time to time: big groups of Central Americans joining together to face the challenges of migration, their numbers providing security against the criminals that stalk the route north.

But this one has drawn the ire of President Trump, who warned on Thursday that he would shut down the southwest border of the United States if Mexico did not halt the group. He also reiterated his threat that unless the governments of Guatemala, Honduras and El Salvador stopped the caravan’s progress, he would suspend foreign aid to them.

“I must, in the strongest of terms, ask Mexico to stop this onslaught —” he posted on Twitter, “and if unable to do so I will call up the U.S. Military and CLOSE OUR SOUTHERN BORDER!”

And for good measure, perhaps with the midterm elections in mind, he took a shot at the Democratic Party, blaming it in part for the caravan of migrants.