Last April, President Donald Trump announced, “We’re going to be guarding our border with the military” until congressional support for funding a U.S.-Mexico border wall can be secured. This led to the deployment of state National Guard personnel to the border. During this deployment, they are acting under state legal authorities (so called “Title 32” authorities, referencing the title in U.S. Code). In response to the April deployment, I wrote a piece, titled The Military, Mexican Border and Posse Comitatus: Four Key Takeaways, which provided general guidance concerning what troops were legally permitted to do on the border.

Trump double-downed on his vision for a more militarized border last week. In the lead up to the midterm elections, and in response to a U.S.-bound immigrant “caravan” fleeing Central American violence, Trump has now ordered active-duty military forces (so called “Title 10 forces”) to the U.S.-Mexican border. Unlike the earlier use of state National Guard personnel – whose deployment to the border continues – the use of federal forces operating under federal control raises two additional issues concerning:

(1) what kind of support the “Title 10 military” can provide to law enforcement; and

(2) rules for the use of force.

1. First, unlike “Title 32” National Guardsmen and women, “Title 10” active-duty military personnel under federal control are largely prohibited from taking an active and direct role in law enforcement activities. And for good reason: The Posse Comitatus Act has been in place since 1878 and prohibits any part of the Army or Air Force from “execut[ing] the laws.” The Department of Defense has extended this prohibition on taking an active role in law enforcement to the Navy and Marine Corps by regulation (but not the Coast Guard). So any direct involvement in law enforcement – think of a search, seizure, apprehension or arrest – would violate the Posse Comitatus Act as well as governing military directives. That’s a no-go.

The military border mission, named Operation Faithful Patriot, is best described as “defense support of civil authorities” where indirect assistance to law enforcement is authorized, but direct assistance is not. Prohibited direct assistance to law enforcement includes using force or physical violence and discharging or using a weapon (except in self-defense). And traditional law enforcement functions to include evidence collection, security functions, and manning and staffing checkpoints are also prohibited based upon existing directives.

Indirect assistance is authorized. This could potentially include providing and operating military equipment, sharing information with Customs and Border Patrol officials, aerial reconnaissance, and detecting and monitoring the immigration flow. But the laws and governing regulations authorizing this support date from the “War on Drugs” in the 1980s. They were passed with the specific goal of easing military restrictions to overwhelmed law enforcement officials in response to illicit drug trafficking entering the United States. So if a federal lawsuit is brought challenging the scope of the military’s activities at the border, it remains unclear how a court would rule on such a challenge when drug trafficking is not remotely the issue.

Relatedly, actions that “further a DoD or foreign affairs function of the United States” are exempt from most of these restrictions. While this provision is not well-defined, stopping a caravan of unarmed immigrants (many seeking legal asylum) is aligned with the Department of Homeland Security’s mission to secure borders and is outside the DoD’s mission to deter war and protect the security of the nation. And it is clearly not aligned with the historical use of the military, particularly since World War II. In light of the above, it is not surprising that the Pentagon initially pushed back on the Trump administration’s desire for the military to play a more direct role in law enforcement. As Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford said Monday, the U.S. military will not be “involved in the actual mission of denying people entry to the United States.”

2. Second, standing rules for use of force (“SRUF”) not standing rules of engagement (“SROE”) will govern this military mission. What’s the difference? As a general matter, ROE govern military operations in environments where host-nation law enforcement and civil authorities are nonexistent or otherwise resistant to U.S. military presence. Rules of engagement involve a more “combat-mindset.” It may even involve a declaration that certain forces are hostile, whether or not the individual poses an imminent threat of death or serious personal injury. Rules of engagement are employed largely outside the U.S. in uncertain environments – think of the ongoing military operation in Afghanistan.

In contrast, rules for the use of force (“RUF”) are based on a law enforcement and self-defense mission and mindset to include this border deployment. It takes into account domestic legal considerations: This includes the Posse Comitatus Act, the 4th Amendment and existing constitutional provisions. Rules for the use of force cannot authorize force in excess of constitutional reasonableness, nor can it declare certain forces hostile. Shifting from a combat “ROE mindset” to a law enforcement “RUF mindset” is critically important for the troops deployed to the border as it will instruct how they approach real or perceived threats. It will require training, leadership, and working through hypothetical scenarios to avoid mistakes.

The decision to use force is necessarily a fact-specific inquiry – and hopefully military forces are far removed from any possible altercation. But consider how this could play out. Trump initially stated that the military should and could “shoot rock-throwing migrants”. While he has since backed away from this assertion, it certainly muddied the waters on what constitutes the legal use of force at the border. Yet it is difficult to imagine a scenario where rock-throwing or a similar activity would provide the legal justification for the use of deadly force. Under existing use of force guidelines, deadly force is only authorized when there is a reasonable belief that the subject of such force poses an imminent threat of death or serious bodily harm. Again, having the proper training and instilling a self-defense mindset to implement the rules for the use of force is absolutely essential to avoid unnecessary casualties. Twenty years ago, Marines were ordered to the Mexican border to help stem the flow of illegal narcotics trafficking. In a tragic case, a Marine killed a young U.S. citizen herding goats at the border. While the exact circumstances of the shooting have been disputed for years, a congressional report highlighted the lack of training on civilian law enforcement measures and the lack of understanding of local conditions.

Perhaps most importantly, we all need to take a step back on the real or perceived “threat.” The immigrants who eventually make their way to the border (still over a month away) include families, children, and people who will be physically and mentally exhausted. They will be in need of food, water, and medical assistance. Thankfully, the military is well-equipped and trained to provide such humanitarian assistance. Hopefully, there will not be any need to make difficult decisions about life and death.

Image: Members of the U.S.military place razor wire along the U.S.-Mexico border on Nov. 2, 2018, in McAllen, Texas. AP Photo/Eric Gay