With the stroke of a pen, President Donald Trump issued an executive order declaring the situation at the southern border a national emergency. Trump also issued an executive order titled “Clarifying the Military’s Role in Protecting the Territorial Integrity of the United States.” This second executive order directs the Secretary of Defense to deliver a plan to “seal the borders and maintain the sovereignty, territorial integrity, and security of the United States by repelling forms of invasion including unlawful mass migration, narcotics trafficking, human smuggling and trafficking, and other criminal activities.” Trump’s second executive order tasks the Secretary of Defense to send as many units or members of the Armed Forces as “appropriate to support the activities of the Secretary of Homeland Security in obtaining complete operational control of the southern border of the United States.”
In a separate statement of America First priorities, the president notes:
“Armed Forces, including the National Guard, will engage in border security, which is national security, and will be deployed to the border to assist existing law enforcement personnel.” (emphasis added)
These executive orders, while not surprising, represent a fundamental shift in U.S. immigration and national security priorities. They also elevate and potentially significantly expand the military’s role at the southern border. Notably, Trump used the verb “assist” in his America First policies, and phrases such as “support the activities of the Secretary of Homeland Security” and “assist the Department of Homeland Security” in the first order. These key terms likely indicate that the military will not displace Department of Homeland Security and civilian law enforcement at the border. On the other hand, by characterizing immigration as an “invasion” and tasking the military with protecting “the sovereignty and territorial integrity of the United States along our national borders,” the second executive order may expand the “military purpose doctrine” beyond historical usage (and inching closer to a law enforcement purpose instead).
Does President Trump have the legal authority to declare a border emergency?
Yes. The president has broad authorities to declare an “emergency” within the meaning of the National Emergencies Act of 1976. Congress has failed to define emergency within that statute. Congress has also failed to provide explicit criteria for when an emergency exists, and national emergency declarations are a rather common tool used by presidents of all parties. Trump’s border emergency declaration is facially legal, despite border crossing numbers that have been plummeting for the past several months.
Courts have generally deferred to the president in making such an emergency determination, and plaintiffs often don’t challenge presidential authority to declare a national emergency. Trump’s 2025 declaration is somewhat analogous to his February 2019 decision to declare a national emergency at the southern border—an emergency Biden quickly rescinded upon taking office.
Declaring a national emergency acts as a key that unlocks the door to 150 statutes across the U.S. code that in turn empower the president with additional authorities. We are in a state of approximately forty national emergencies as of this writing, with some emergencies dating back to the Carter administration. While the president is given a “legal key” to declare an emergency, they must still comply with the statutory text of each “legal door” that they seek to open.
For example, Trump is using a specific military construction statute, 10 U.S.C. § 2808, that diverts money away from military construction projects to construct a border wall. But to activate this authority, the president must comply with section 2808’s statutory text. Section 2808 authorizes military construction projects “that are necessary to support [] use of the armed forces” in cases where a national emergency requiring use of the armed forces has been declared. Unlike his February 2019 executive order that tapped into similar authorities, Trump has now issued a second executive order that closely links the U.S. military’s mission with border security and “protecting the sovereignty and integrity of the U.S. along our national borders.” By linking the military’s mission so closely to border security, the Trump administration appears to anticipate legal challenges that the use of military construction funds for border wall construction is not necessary to support the armed forces, but I am not sure the president’s strategy will be successful.
After all, Trump faced similar legal challenges in 2019 that he struggled to overcome in federal court. The Ninth Circuit found that the use of Section 2808 authority to construct the border wall was not aligned with the text of 2808. According to the Ninth Circuit, the challenged construction projects were not necessary to “support the use of the armed forces with respect to the national emergency on the southern border, nor were they military construction projects.”
Time will tell if the new linkage between the military mission and border security is convincing to a federal court. In 2019, states and environmental groups were able to establish standing by asserting environmental injuries related to the border wall construction. I’m skeptical that the Ninth Circuit, which has federal jurisdiction over challenges in California and Arizona, will fully disregard the statutory text and defer to the president on this matter.
Are there relevant legal or operational constraints on the president’s actions?
Yes, there are several. But many of them would require judicial intervention or congressional action.
First, the Posse Comitatus Act. Absent an Insurrection Act invocation or other exception, the president is prohibited from using active duty, federal military forces in direct support of law enforcement within the United States. This restriction is due to the Posse Comitatus Act of 1878. The modern version states:
Whoever, except in cases and under circumstances authorized by the Constitution or Act of Congress, willfully uses any part of the Army, the Navy, the Marine Corps, the Air Force, or the Space Force as a posse comitatus or otherwise to execute the laws shall be fined under this title or imprisoned not more than two years, or both.
So active duty, federal military forces cannot provide direct law enforcement support to civil authorities in the form of searches, apprehensions, arrests, or seizures. The Posse Comitatus Act also prevents military members from engaging in interviews, interrogations, or questioning potential witnesses or suspects. Posse Comitatus Act prohibitions do not apply to National Guard forces operating under a state active duty or Title 32 (federal funding) status. Nor does the Posse Comitatus Act apply to the U.S. Coast Guard.
These prohibitions are reinforced in Pentagon instructions and doctrine (and long-standing Justice Department Office of Legal Counsel (OLC) opinions). But federal military forces can still provide indirect support to law enforcement via “Defense Support to Civil Authorities.” Indirect support may include detection, monitoring, and communication as well as logistical and other administrative support.
Significantly, courts have held that Posse Comitatus Act restrictions do not apply to federal troops’ actions taken primarily for a “military purpose”—even if such activities benefit law enforcement. The military purpose doctrine authorizes incidental assistance to law enforcement—this could act as a bit of a legal workaround to the general prohibition of direct law enforcement support. The military purpose doctrine is shaped by case law, and its status is reinforced within Pentagon instructions. The military purpose doctrine authorizes “actions taken for the primary purpose of further a Department of Defense or foreign affairs function of the United States.”
Traditionally, the military purpose doctrine has authorized the military, without running afoul of Posse Comitatus Act restrictions, to maintain order, security, and discipline on or near a military base. But deciphering what actions fall within the military purpose doctrine is notoriously difficult, with the OLC opining that distinguishing between military and law enforcement functions when dealing with terrorism is “no easy task.” Trump’s second executive order may be taking advantage of this legal gray area by directly connecting the military’s role with safeguarding the territorial integrity of the nation. Trump’s reference to an “immigrant invasion” and his designation of certain cartels as foreign terrorist organizations further blur the lines between law enforcement and traditional military activities that fall under the umbrella of the military purpose doctrine.
At least one OLC opinion from 2001 advised the Pentagon that the Posse Comitatus Act imposed no constraint on the use of military forces for the “performance of military functions” such as operations against “international or foreign terrorists operating within the United States.” While this 2001 OLC memo was superseded in 2008 by a new OLC memo that circumscribed the military’s role in domestic operations, one can easily imagine a 2025 Trump OLC that favors the more robust, 2001 approach to Posse Comitatus Act restrictions. It may be one reason behind the foreign terrorist designation of certain cartels. But, in that case the use of the military would, as a threshold matter, need to be addressed to that particular threat (not migrants in general).
Second, Fiscal Law. Broadly speaking, within national security law, Congress has the power of the purse and the president has the power of the sword. Article I, Section 9 of the Constitution vests in Congress the power to provide that “[n]o Money shall be drawn from the Treasury, but in Consequence of Appropriations made by Law.” This constitutional provision is implemented by 31 U.S.C. § 1301(a), which states that appropriations may be used only for the purposes set forth in the statute. Congress already provides great specificity in its annual military spending bills governing how, when, and where military appropriations are spent. Within the annual spending bill, there is a recurring provision (Section 8005) that authorizes the Pentagon to transfer up to $6 billion provided that the funds are being transferred for (1) higher priority missions; (2) based upon unforeseen military requirements; (3) whose purpose has not previously been denied by Congress.
The upshot: the Trump Administration has to meet these transfer authority requirements before using the money to fund troop deployments that are outside the Fiscal Year 2025 defense spending bill. And the $6 billion recurring limit may seem like a lot. It’s not. As Professor Chris Mirasola has noted, this figure represents far less than 1% of the Pentagon budget. The president may have a bit of fiscal runway to execute his immigration vision, but the runway is short. At some point Congress has to act or the troops go home.
Third, Environmental Law. Core environmental laws apply to any border wall construction. And environmental claims can be asserted alongside the Administrative Procedure Act to provide a basis for a cause of action and help establish an injury under Article III standing. Indeed, environmental groups had considerable success suing Trump in 2019 using existing environmental laws such as the Endangered Species Act. What’s more, environmental injuries can assist plaintiffs in establishing an injury-in-fact, a key requirement for Article III standing analysis. Ruling for the State of California, the Ninth Circuit held that California “has an interest in the natural resources of its state … and dozens of endangered or threatened species will be at serious risk due to construction.”
To be sure, several environmental laws have national security exemptions, and Trump has declared a “national energy emergency” with an eye toward bypassing environmental laws. But the national emergency at the southern border is silent on waiving environmental laws. And the energy emergency is distinct from the border emergency, and is designed to facilitate energy production—a completely different focus than border security. It remains unclear how the energy emergency is linked to the border emergency, if at all. Further, state environmental laws and regulations continue to apply to any border wall construction (California has some of the strongest environmental laws in the country).
Fourth, Military Preparedness & Operational Risks. There are significant operational risks for any military involvement in immigration and law enforcement. Further, existing law prohibits the provision of miliary support to civilian law enforcement agencies “if the provision of such support will adversely affect the military preparedness of the United States.” Other operational considerations cannot be wished away. The military’s core job is to fight and win the nation’s wars and most military members are trained for combat—not law enforcement missions. Deploying military forces to the border diverts resources, impacts readiness, and detracts from the core military mission. And the state National Guards just redeployed from supporting the COVID-19 response, the largest domestic military deployment in recent history. This is a fatigued, overburdened, and stressed force that is facing historic recruiting and retention challenges; it is hard to see a scenario where deploying to the border makes the force healthy and ready for whatever national security challenges come our way.
Is it significant that the president stated the military will “secure our borders against threats of invasion?”
Yes. The choice of words—“invasion”—is deliberate for constitutional reasons and could be laying the groundwork for a more expansive miliary role in immigration law enforcement. During his inauguration speech, Trump also directed the military to “send troops to the southern border to repel the disastrous invasion of our country.”
Article I, Section 10, Clause 3 of the Constitution states that “[n]o state shall, without the Consent of Congress, … engage in war, unless actually invaded, or in such imminent Danger as will not admit of delay. Further, the Constitution’s Guarantee Clause in Article IV states that the “United States shall guarantee to every state in this Union a republican form of government, and shall protect each of them again invasion …”
Independent of Congress’ power to declare war, as Commander in Chief, the president has the duty and power to “repel sudden attacks.” Extending this authority to immigrants entering the country is far outside our historical understanding of this Clause and the plain meaning of what constitutes an “invasion.” One dictionary defines invasion as “the act of invading, incursion of an army to conquest or plunder.”
And immigration as invasion is not aligned with the original meaning and founders’ understanding of the term—James Madison stated that an “[i]nvasion is an operation of war” in response to claims that the Alien and Sedition Acts of 1798 were authorized by Article IV.
Texas Governor Greg Abbott made a similar argument earlier this year when he signed into law S.B.4, which expanded Texas’ role in immigration policy and enforcement. This line of thinking—immigration is equivalent to invasion—was forcefully rejected in United States v. Abbott. Judge Ezra meticulously walked through the definition of invasion, concluding that “surges in immigration do not constitute an ‘invasion’ within the meaning of the Constitution.” Even the most uncommitted textualists should be wary of equating immigration with invasion.
Going forward, I will be looking to see if the president attempts to tie the invasion rhetoric with the outer contours of the military purpose doctrine discussed earlier. If, indeed, the president is equating immigration to invasion to set the stage for a broader expansion of military authority to halt immigration at the border, we are in uncharted territory. Indeed, such an approach would mark an extraordinary expansion of the military’s role in immigration law enforcement. But the president has shown a willingness to expand earlier, somewhat obscure authorities—such as 502 (f) authorities—when he requested outside National Guard units to deploy to Washington, D.C. in the summer of 2020.
Did the President invoke the Insurrection Act?
No. The Insurrection Act authorizes the president to deploy active-duty armed forces and to federalize the National Guard, acting as the most important exception to the Posse Comitatus Act. Despite its name, the Insurrection Act does not actually require an insurrection—it can be used, for example to “enforce federal law or suppress a rebellion against U.S. authority.” Thankfully, President Trump is not using Insurrection Act authorities in his opening immigration salvo given the profound implications that would mean for the American military and unnecessary escalation, but this does not mean that he won’t do so eventually. Trump appears to be relying on Title 32 federal funding authorities to deploy the National Guard and the 8005 authority to deploy federal military forces. But the president’s bellicose language, reference to the Alien Enemies Act of 1798, and call to “secure our borders against threats of invasions” could set the stage for a broader effort to use federal military forces for law enforcement or federalize National Guards in several states. Indeed, the first executive order requires the Secretaries of Defense and Homeland Security to provide a report within 90 days on “whether to invoke the Insurrection Act.” Stay tuned.
Where do we go from here?
Going forward, I will be watching responses to the executive orders from three different areas, all leading to additional questions and some clarification.
First, I will be closely watching how the Department of Defense and its military leaders interpret these executive orders and operationalize the directives for a coherent military plan. All eyes will be on the Secretary of Defense and Northern Command (“USNORTHCOM”), which are tasked to deliver and execute a plan by the end of the month on border security. How does the military contemplate its role in executing the president’s vision in immigration enforcement?
Second, I anticipate litigation from environmental groups to challenge the legality of the military construction at the southern border. Will these arguments countering the construction be successful, similar to the 2019-2020 litigation?
Finally, I will be watching Congress—particular the fiscal hawks—to see how they respond to these deployments. Trump enjoys the slimmest of majorities in the House, and at some point Congress will have to act to provide supplemental funding beyond the recurring transfer authority. Will they?