STARR COUNTY, Tex. — The soldier stood on a cliff above the swift green waters of the Rio Grande, peering into the brush with binoculars, an M-4 carbine rifle at his chest, a 9-millimeter pistol in a holster low on his thigh. The rainstorm was over, but a thick layer of mud lined the bottom of his combat boots.

He was one of 250 Texas National Guard troops stationed on the border with Mexico, part of President Trump’s latest plan to stanch the flow of immigrants entering the country illegally. The troops at the observation post chatted very little. They stared into the brush, took a few steps to change their position, then stared some more.

Then it happened — one of the soldiers saw a raft in the river. The troops got on their radio and summoned the Border Patrol down below, fulfilling, military officials said, one of the primary missions of the National Guard’s controversial mobilization on the southern border: to observe and report.

On Tuesday, the Guard allowed a group of journalists to shadow some of its troops on the border, an early look at the new deployment that follows a surge in illegal crossings into South Texas. The Department of Homeland Security said more than 37,000 people were detained in March. Though the flow of asylum seekers into Texas regularly goes up in the springtime, detentions this year were three times the levels seen in March of last year.