As a widely publicized caravan of thousands of migrants makes its way through Mexico toward the United States and sets off a new round of fury from the White House, President Donald Trump has threatened to use the military if necessary to repel them.

But the reality is that for now, Trump can do little more than complain.

Most of the ideas Trump has floated in response to the caravan seem unlikely to flourish as U.S. policy. Hes vowed to cut off aid to Honduras, El Salvador and Guatemala, where most of the travelers are from. But that would require congressional approval and runs the risk of exacerbating the regional instability that caused many to leave in the first place. Hes threatened to shut down the U.S.-Mexico border, but that would be disastrous for trade and for the roughly 500,000 people who legally cross into the U.S. every day for work, school, shopping and tourism.