President Trump has called for imposing tariffs on Mexico, the administration’s latest attempt to slow the stream of migrant families who have been arriving at the southwest border by the thousands each day.

It is just the latest move by the president to rein in illegal immigration after a series of executive orders, regulations, ramped-up border security and diplomatic threats have failed to dissuade Central Americans, mostly fleeing poverty and violence, from journeying north. In fact, migrant families have continued to arrive in unprecedented numbers.

Nearly 110,000 people were apprehended at or near the border last month, the largest number since 2007. On Wednesday alone, the Border Patrol in El Paso encountered a group of 1,036 migrants, the largest single group ever.

Since his days on the campaign trail, the president has made unauthorized immigration central to his agenda, framing it as a threat to national security that creates unfair competition for American workers. Thursday’s announcement that the government would impose across-the-board tariffs on Mexico until that country made significant reductions in the number of people entering the United States prompted a number of warnings: that it would impede manufacturing supply chains, jeopardize a new North American trade agreement, and impede cooperation with Mexico over immigration issues.