On Tuesday the White House approves troops stationed at the Mexico border to use lethal force and engage in some law enforcement roles, if necessary. Many legal experts have cautioned that this may run afoul of the Posse Comitatus Act.
This new "cabinet order" was signed by John Kelly, the White House Chief of Staff, not President Trump. Per Military Times, it allows “Department of Defense military personnel” to “perform those military protective activities that the Secretary of Defense determines are reasonably necessary” to protect border agents, including “a show or use of force (including lethal force, where necessary), crowd control, temporary detention. and cursory search.”
— Tara Copp (@TaraCopp) November 21, 2018
There are activities, such as crowd control and detention that may run into direct conflict with the 1898 Posse Comitatus Act. Legal experts suggest that if this act's limitations are crossed, it could create a fundamental shift with the way the U.S. military is used.
Per Military Times:
The Congressional Research Service, the non-partisan research agency for Congress, has found that “case law indicates that ‘execution of the law’ in violation of the Posse Comitatus Act occurs (a) when the Armed Forces perform tasks assigned to an organ of civil government, or (b) when the Armed Forces perform tasks assigned to them solely for purposes of civilian government.” However, the law also allows the president “to use military force to suppress insurrection or to enforce federal authority,” CRS has found.
Military forces always have the inherent right to self defense, but defense of the border agents on U.S. soil is new. In addition, troops have been given additional authorities in previous years to assist border agents with drug interdictions, but the widespread authorization of use of force for thousands of active-duty troops is unique to this deployment.
Each domestic deployment of troops to any of the 50 states or U.S. territories is governed by Chairman of the Joint Chiefs of Staff Instruction 3121.01B, “Standing Rules of Engagement, Standing Rules for the Use of Force by U.S. Forces.” Two annexes, L and N, are specific to Defense Department missions in support of civilian authorities.
However, each mission is unique, and the standing rules for the use of force can be adjusted except for the limitation against active-duty U.S. forces conducting law enforcement on U.S. soil, which is enshrined in the 1898 act.
There was already a moment before the mid-term elections where President Trump said that migrants throwing rocks could be considered the use of force. At this point President Trump threatened to use firearms to retaliate. At a later time President Trump backed off of the rifles comment and that Customers and Border Protection would take over in that type of scenario.