Plans for the military to participate in the detention and deportation of 9 million immigrants will foreseeably undermine the well-being of current service members and future veterans while damaging the morale, reputation and readiness of our nation’s armed forces.   Americans have historically rejected attempts to muddle the distinction between military and civilian authority. The Bill of Rights was designed to guard against this. The 1878 Posse Comitatus Act (literally “the power of the nation”) forbids military participation in law enforcement without congressional approval. As a former career Army medical officer and a retired Veterans Affairs psychiatrist with decades of clinical and policy experience, we speak out against violating this core American principle.   We have learned firsthand that ordering service members to perform policing duties in ambiguous situations without adequate training, leadership and support creates serious risks for those they detain and for the service members themselves.   Rushing to set up and operate detention facilities and their required logistical and medical support inevitably precipitates chaos and confusion. Foreseeable challenges in the proposed mission include:  — There is an insufficient number of experienced military police brigades to conduct the proposed mission. Infantry, armor and field artillery units could be assigned to augment the workforce but lack the necessary training.  — Encampments have restrictive rules of force as demonstrated during foreign peacekeeping missions. Military members may be exposed to violence, chaos and flashpoints such as detainee-on-detainee attacks or attempts by American citizens (individually or as private militias) to intervene as vigilantes.   — Research shows that service members assigned to humanitarian missions, such as providing medical support during COVID-19, suffered elevated levels of burnout, worsening mental health and increased thoughts of suicide.  — At a time when our military faces global threats and is challenged by a recruiting crisis, this mission would further stress our armed forces. — Converting existing military bases to confine detainees would disrupt normal military operations and adversely impact readiness.  We view immigrant encampments as humanitarian operations intended to strengthen national security. Twenty years in Iraq and Afghanistan have shown that effective humanitarian operations require expert planning and execution.   Even correctional staff who have worked effectively in prisons are unprepared for the challenges to be encountered in large encampments of mixed genders, ages, cultures and English-speaking ability. Staff may reasonably fear for their own safety. This predictably leads to unsafe and unethical behavior. Abu Ghraib provides a glaring example. A mix of active duty, Reserves troops and private contractors operated within an inadequate facility in which lines of responsibility and authority were blurred. Ultimate accountability for degrading and injurious behavior towards detainees fell on front-line service members who lacked appropriate training, leadership and support.  Military medical staff, while essential in detainee operations, may be de-skilled in such situations. This was sadly demonstrated when military psychologists were drawn into interrogation efforts at Guantánamo. Intending to save lives and ensure national security, they ultimately violated the basic clinical principle of doing no harm to those under their care. Appropriate training and field experience in police operations is essential in maintaining the dignity and self-respect of both the detainees and those detaining them. Without adequate preparation, service members (including medical staff) will be put at significant and unnecessary risk. The men and women of our all-volunteer force have pledged their lives to defend our people and our values. They are an ethical profession driven by a high standard of service to others. Ordering them to detain millions of men, women and children in makeshift facilities exposes them to ethical and moral challenges which may violate basic beliefs and standards. Those who feel they have perpetrated, failed to prevent, or merely witnessed atrocious acts are at risk for intense and persistent feelings of shame and guilt. These moral injuries often lead to social isolation, family breakdown, unemployment and homelessness. They may also complicate posttraumatic stress disorder, depression, substance use, traumatic brain injury and suicidality. As health professionals and fellow citizens, we have a duty to foresee and prevent this downward spiral. To assemble the required workforce, other government agencies would be required to detail personnel. Tasking staff to operate outside their primary occupations, organizational structures and agency cultures is problematic. Hastily deployed augmentees would be further challenged by the complexity inherent in detaining adults, children, the elderly and entire family units. While medical support could be contracted to private companies that supplement the Bureau of Prisons, these contractors have been widely criticized for failing to maintain basic standards of care. The proposed encampments will require even higher levels of care to meet a broad range of preexisting medical needs and prevent major outbreaks of infectious diseases. Military medical systems are already stretched thin. Reassigning personnel to detention centers would profoundly degrade normal military medical operations and cripple readiness. The VA medical system is struggling with a budgetary crisis which threatens its ability to back up the military in war or the community in disaster. It would be unwise (and unethical) to encumber VA with a task which undermines its mission. The nation can neither ensure its security nor maintain its fighting force by violating the principles which define the relationship between its military and civilian populations. This social contract is foundational to our democracy and enshrined in our laws and culture. Beyond this, we have learned painful lessons about “going to war with the Army we have.” We do not need to repeat this mistake.    Stephen N. Xenakis, M.D., Brigadier General (Ret.), U.S. Army is a psychiatrist and a member of the executive board of the Center for Ethics and Rule of Law. Harold S. Kudler, M.D., is a retired Department of Veterans Affairs psychiatrist who served as VA’s chief consultant for mental health and co-led development of the VA/Department of Defense Clinical Practice Guideline for Management of Posttraumatic Stress Disorder.