The plan would put a strip along the southern border under Pentagon control, allowing the military to detain migrants for trespassing on a military base until Border Patrol agents could arrest them.

The Trump administration’s major military mission at the southern border focused on reducing immigration and drug flows has already cost taxpayers more than $300 million, according to sources briefed on data from the Defense Department comptroller — even as the administration has vowed to slash the size of government and cut 8 percent from the department’s budget.

It marks an escalation in the president’s use of the military to facilitate his sweeping immigration agenda. A 60-foot wide strip of land along three southwestern border states will be placed under the jurisdiction of the U.