The White House authorizes troops stationed at the border to engage in some law enforcement roles, and use lethal force if necessary.

The order was signed by White House Chief of Staff John Kelly, not President Trump. It allows “Department of Defense military personnel” to “perform those military protective activities that the Secretary of Defense determines are reasonably necessary”

https://www.militarytimes.com/news/your-military/2018/11/21/white-house-approves-use-of-force-some-law-enforcement-roles-for-border-troops/