Troops are not allowed to apprehend immigrants like border agents do. For the most part, they are laying barbed wire barriers and erecting tent facilities for themselves and the Border Patrol. The only armed troops are military police, who are there to protect the encampments where troops are staying. While some troops will wear body armor, they will do so only for self-defense and will not directly interact with immigrants.
Trump has said the troops are defending the border and called them a "human wall." His critics have called it a campaign stunt designed to drive fear of immigration ahead of the mid-term elections. Military officials are using it as a readiness exercise, hoping troops will at least get training at the border that they can use on future deployments.
One U.S. Customs & Border Protection official said border agents are happy to have support for jobs like rolling out barbed-wire.
"I think they are happy that the military will be doing the work that agents don't want to do," said the official, speaking on the condition of anonymity.
Most of the troops are in Texas, including in the Rio Grande Valley, the southern border's most heavily trafficked section. Others are spread along other sectors in California and Arizona. Some sectors, such as El Paso, have said they don't know if or when they will have troops coming to support them.
"Our service members are performing duties along the Southwest Border in support of Customs & Border Protection and will continue to do so until otherwise directed by our senior leaders, to include the commander-in-chief," said Mike Kucharek, chief of integrated communications for U.S. Northern Command, which is overseeing the mission.
The latest figures on the migrant caravan predict somewhere between 3,500 to 5,000 Honduran migrants have made it to Mexico City and plan to travel north to the United States sometime over the next month. It is unlikely all of the caravan would cross the southern border in one day, but even if they did, the number is not expected to overwhelm border agents. In September, the latest month for which data is available, border agents arrested or denied entry to 1,685 undocumented immigrants per day on average.
CBP is the largest federal law enforcement agency in the United States, with more than 40,000 sworn officers and more than 60,000 total employees.
Julia Ainsley is a correspondent covering the Department of Homeland Security and the Department of Justice for the NBC News Investigative Unit.
Courtney Kube is a correspondent covering national security and the military for the NBC News Investigative Unit.