The Trump administration plans to grant U.S. troops on the Mexican border the authority to use force to help protect Border Patrol officers, defense officials said Tuesday, a significant widening of a mission already criticized as politically motivated rather than a national security priority.

President Trump is expected a sign an executive order giving armed troops the authority to intervene if U.S. personnel are endangered by migrants trying to cross the border, the officials said. The order doesn’t rule out use of deadly force.

The order could face legal challenges to determine if it violates a U.S. law that bars active duty military from conducting domestic law enforcement functions. The Posse Comitatus Act, first passed in 1878 and amended several times, is intended to limit the government’s ability to use military personnel within the United States except in natural disasters, terrorist attacks and other national emergencies.

Roughly 5,900 active duty soldiers and Marines were sent to border posts in California, Texas and Arizona shortly before the Nov. 6 election in one of the largest such deployments in decades. Most of the troops are unarmed and restricted from interacting with migrants, and critics — including some former senior military officers — condemned the operation as using active duty troops for partisan gain.

Trump’s expected expansion of their role is likely to deepen questions into whether he is trampling longstanding practices on the legal use of the military on U.S. soil, and raises the risk that a confrontation with unarmed migrants could escalate into deadly violence.

It also could lengthen the time at least some of those troops must spend away from home. Commanders previously have said they plan to withdraw all troops by Dec. 15 unless an extension is ordered. After news reports suggested that some troops could be withdrawn this week, the Pentagon pushed back and said some may be transferred to other border posts.

"We are continually assessing our resources and refining requirements,” the Army command based in San Antonio, Texas, that is overseeing the operation said in a statement Tuesday. “We may shift some forces … to engineering support missions in California and other areas. No specific timeline for redeployment has been determined.”

The deployment will cost about $72 million through Dec. 15, but the final cost “has yet to be determined and will depend on the total size, duration and scope” of the operation, according to Col. Rob Manning, a Pentagon spokesman.

So far, the troops on the border have been authorized only to protect other military personnel. Expanding that authority to help protect Border Patrol agents, who are also armed, would likely involve military police units, officials said.

Trump suggested last month that the military might open fire on anyone who threw rocks at them on the border, a statement he later amended, saying migrants who threw rocks would be arrested and prosecuted, not shot. The Border Patrol has long faced criticism that officers have used excessive force, including killing unarmed individuals.

Democrats stepped up their criticism of the deployment Tuesday, accusing Trump of deploying the military unnecessarily for partisan gain before the election by exaggerating the threat from caravans of Central American migrants. Several thousand people have arrived in Tijuana, near the the San Ysidro border crossing, but more are en route and are expected to seek asylum.

“It was not a respectful use of our military to take service members away from their duties and send them to the border as politicized props, and President Trump should not have done it,” said Rep. Adam Smith (D-Wash.), who is likely to take over as chairman of the House Armed Services Committee in January. “They should all be returned to their regular duties.”

More:

https://www.latimes.com/nation/la-na-pol-border-troops-20181120-story.html