The U.S. chairman of the Joint Chiefs of Staff and the commanding general of Northern Command may soon have to make the most important decision of their careers: whether to obey an order from their commander in chief to invade Greenland. 

The answer should be obvious: No. But the evident willingness of most of the American military leadership to execute President Trump’s orders to kill alleged drug smugglers on the high seas, which I and most commentators consider unlawful, and to seize the Venezuelan president Nicolás Maduro from his compound makes the answer to this question less certain.

It should not.

Although the President commented in his remarks at the World Economic Forum in Davos that he has decided—for the moment—not to use military force to acquire Greenland, he stopped well short of permanently renouncing the use of military power to pursue his goals, in Greenland or elsewhere. The moral and legal challenges for American military leaders, therefore, remain alive and urgent.

The president had at least a plausible reason for ordering American armed forces to raid the territory of Venezuela to arrest Mr. Maduro. Federal grand juries in New York had twice indicted Mr. Maduro for his alleged role in distributing drugs into the United States. Thus, he could reasonably be treated as a fugitive from justice. The military promptly transported him to the custody of civilian authorities in New York to be brought before a federal court to answer the charges.

In addition, Mr. Trump could invoke the capture of Manuel Noriega as a precedent, established by the first President Bush in 1989, when the United States launched a full-scale invasion of Panama to seize the indicted president of that country to face similar charges.

But Greenland is another matter altogether.

Flouting International Law

At least since St. Augustine, the concept of “just war” has depended on whether the reason for using force is legitimate, that is, whether there is jus ad bellum. Any use of military power that is not justified by an acceptable reason is, therefore, unjust and immoral.

As a matter of moral principle, the result is clear: No member of the military, including especially the senior leaders who would be responsible for receiving and transmitting such an order, may obey it.

On Jan. 19, three American cardinals who had recently returned from meeting with Pope Leo XIV issued an extraordinary declaration on the “morality of U.S. foreign policy.” Undeniably reacting to President Trump’s threats to seize Greenland by force, the cardinals asserted: “We renounce war as an instrument for narrow national interests and proclaim that military action must be seen only as a last resort in extreme situations, not a normal instrument of national policy.”

In a recent interview with the BBC, Timothy P. Broglio, the archbishop for the Military Services, stated that it “would be morally acceptable” for U.S. troops to disobey orders that violated their conscience, including an invasion of Greenland. He explained the consequence of this well-settled principle of Western morality for the men and women in uniform for whom he is the principal spiritual counselor: 

It would be very difficult for a soldier or a Marine or a sailor to by himself disobey an order…. But strictly speaking, he or she would be, within the realm of their own conscience, it would be morally acceptable to disobey that order, but that’s perhaps putting that individual in an untenable situation—and that’s my concern.

Although this dilemma would be particularly onerous for the rank-and-file members of the military, it also frames the duty to which the senior military leaders are called: to advise the president that they will not obey any order that purports to require them and their subordinates to commence an immoral war. 

This is not merely a moral principle but also a rule of law that senior generals and admirals, in particular, are obliged to honor. 

It has been settled since the Nuremberg trials after World War II that “crimes against the peace” and “waging an aggressive war” are not just political abuses but violations of international law. Because they are crimes, both the political leaders who plan and direct an unprovoked attack on the territory of another country and the military subordinates who carry out the orders can be tried by international tribunals and punished.

Led at Nuremberg by the chief American prosecutor, Supreme Court Justice Robert H. Jackson, the Allies established as an essential predicate of the new international order the enforceable principle that even the strong cannot act with impunity to impose their will on their neighbors. Leaders who flout this principle of international law act at their peril.

Nuremberg firmly established that the mere desire to add to a nation’s territory—or lebensraum, as the Nazis called it—is not a legitimate justification for forcibly seizing another country’s land. It makes no difference whether the land is adjacent Poland (or adjacent Ukraine) or nearby Danish Greenland.

The verdicts at the principal trial in Nuremberg led to the conviction and, in several cases, the execution of Hitler’s political advisers and military leaders who had proposed or implemented this policy of military expansionism.

Nevertheless, President Trump’s threats that the United States can acquire Greenland “either the easy way or the hard way” raised the possible use of military power to occupy Greenland, the territory of a NATO ally with which we are, of course, at peace. His influential aide Stephen Miller has been unusually brazen in declaring in a CNN interview that, in his view, the United States may do whatever it has the military power to do:

We live in a world in which you can talk all you want about international niceties and everything else…. But we live in a world, in the real world…that is governed by strength, that is governed by force, that is governed by power.

Without Justification

We have not yet seen reports revealing what American military leaders think about the threats to order them to take Greenland. 

But two points should be clear. One is that invading Greenland would constitute a war crime for which civil and military leaders can be held accountable. The United Nations Charter, which the United States has adopted as a binding treaty, expressly declares in Article 2: “All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state…”

Furthermore, in the treaty under which the United States purchased the (now-U.S. Virgin Islands from Denmark in 1917, the United States expressly relinquished any territorial claims on Greenland. Not surprisingly, several of America’s closest NATO allies have unambiguously declared that any American effort to seize Greenland cannot be tolerated:

Security in the Arctic must therefore be achieved collectively, in conjunction with NATO allies including the United States, by upholding the principles of the UN Charter, including sovereignty, territorial integrity and the inviolability of borders. These are universal principles, and we will not stop defending them.

Some NATO allies have placed troops there to discourage this foolish and unlawful adventurism.

Thus, unlike some of Mr. Trump’s other recent military adventures, there is not even a fig leaf of justification for attacking the Danish territory of Greenland. Any order to do so would invite American military leaders to commit an international crime.

Second, American military law has recognized since at least the Civil War that members of the military have not only the right but the duty to refuse to obey an unlawful order, including the order to commit a war crime. Despite the recent effort by Defense Secretary Pete Hegseth to try to punish Senator Mark Kelly, Democrat of Arizona, for pointing out this principle in the context of the homicidal drug interdiction program in the Caribbean, the honor of the officer corps rests on the willingness to defy unlawful orders. As the Nazi generals learned, it is no defense to lament: “I was just following orders.” 

For rank-and-file soldiers, the notion that a “superior’s order” is automatically an absolute defense against a charge of war crimes was also firmly laid to rest by the Nuremberg war crimes tribunal. During those trials (and ensuing trials that indeed have continued in Germany virtually until the present day), soldiers accused of committing war crimes or crimes against humanity have been unsuccessful in pleading that they were “only following orders.”

This principle is part of American military tradition and American criminal law. A member of the military has no excuse for obeying an order when a person “of ordinary sense and understanding would know the order to be unlawful.”

Current American military doctrine recognizes that commanders not only have an obligation to avoid issuing unlawful orders but also to train their subordinates to question and, if necessary, disobey unlawful orders. Thus, the official “Commander’s Legal Handbook 2025” summarizes the controlling legal principles for military leaders:

Train Soldiers not only to follow the LOAC [law of armed conflict] but also how to respond to unclear or clearly illegal orders. Troops who receive unclear orders must insist, respectfully and tactfully, on clarification. The superior should clarify the order and ensure all understand there is no intent to order a LOAC violation.

If the superior insists on the illegal order, the Soldier has a duty to disobey that order and report the incident to the next superior commander, military police, CID, nearest judge advocate, or local inspector general. Obedience to orders is not a defense to war crimes. A Service member may still be prosecuted for committing a war crime even if they were only following orders (emphasis added).

Nor does it make a difference that it would be the president himself ordering his senior commanders to engage in the international war crime of aggression and breach of the peace. The Supreme Court has repeatedly emphasized that the president’s power as commander in chief is not unbounded. As Justice Jackson put it when the court struck down President Truman’s seizure of U.S. steel mills during the Korean War, it is wrong-headed to think that the president’s role as commander in chief “vests power to do anything, anywhere, that can be done with an army or navy.”

Judicial and Legislative Constraints

Instead, both by treaty and by statute, the options legitimately available to the president and, therefore, the troops under his command are limited.

Although the Constitution makes the president the field commander of American military forces (even though it is hard to equate Mr. Trump with General George Washington as commander in chief), the Constitution reserves to Congress the responsibility for determining how the members of the armed forces conduct themselves. Article I expressly confers on Congress the authority “To make Rules for the Government and Regulation of the land and naval Forces.”

In exercising this power, Congress has made it a federal crime for any American, including members of the armed forces, to engage in “war crimes” as defined by treaty or general principles of international law. Congress also has enacted the Uniform Code of Military Justice, which specifically constrains the actions of American military personnel, including generals and admirals. That code, among other things, prohibits unjustified assault and killings.

The code is explicit in directing that military personnel are only obliged to obey “a lawful command of that person’s superior” and to comply only with a “lawful general order” or “any other lawful order issued by a member of the armed forces, which it is his duty to obey” (emphasis added).

The president lacks the power to direct the armed forces to ignore these constraints. John C. Dehn, who served in the U.S. Army for over 23 years and for over 15 years as an Army judge advocate, noted this clearly in 2018:

[A] President does not possess constitutional power to override congressional regulation of the military, particularly in matters of military discipline. So long as the law of war component of [the UCMJ] remains unchanged, no President may order or authorize war crimes or most other law of war violations that entail or result in a UCMJ offense.

White House officials and civilian and military leaders in the Pentagon should keep the Nuremberg principles and criminal laws in mind. While they hold all the trump cards for the moment, they and their political supporters will not always be in power. Sooner or later, moreover, they will want to travel to foreign countries, where international arrest warrants may render them liable to the penalties of international law.

The best way to forestall a catastrophic rupture in the Western alliance and the post-WWII international order is for the American military leadership to make clear that an order to invade Greenland would be met with a polite but resolute “No, sir.”

Philip Allen Lacovara is the former Deputy Solicitor General of the United States for criminal and national security matters, Counsel to the Watergate Special Prosecutor and President of The District of Columbia Bar.