Officials say CNN is working out the final details, including where the troops are coming from and their specific tasks.
Several government officials say it is expected that the Pentagon will pay for the bulk of the mission through active deployment. It is not yet clear if the Department of Homeland Security will pay the Pentagon.
The troops are there to support the civil authorities, they are not expected to be in regular contact come with migrants. If the troops carry weapons, they serve only for self-defense.
These officials say the White House's Pentagon has been increasingly pressured to send troops to the southern border, especially after the President repeated the use of the military. However, the operation is expected to abide by applicable law, which prohibits the routine use of troops in law enforcement functions.
The troops will not carry out any deadly operations to stop the migrants. Instead, it is expected to provide fences, wall materials, and other technical support at several key points along the border, where it is believed that migrants might try to cross them.
The troops will also provide tents and medical care for the border authorities in these areas. The troops retain the right to self-defense, but border police will still be those who physically stop illegal migration, officials said.
President Trump hinted at the forthcoming efforts with a tweet on Thursday morning and said, "I'm bringing the military out for this National Emergency."
The troops are expected to be in position next week.
This deployment is separate from an ongoing National Guard along the border to assist the border authorities with technical assistance. This mission has about 4,000 authorized people, but the Pentagon has only 2,100.