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President Trump told reporters aboard Air Force One that the United States would acquire Greenland “whether they like it or not.” He offered Denmark the choice between “the easy way” or “the hard way.” This forced American constitutional law to confront a question that has stayed theoretical for seventy-six years: what stops a president from ordering an attack against a NATO ally?
The legal answer is clear. But the practical answer—what would happen if the president issued that order—turns out to be far messier than most people assume.
Denmark’s Prime Minister has already warned that any U.S. military move on Greenland would mean “the end of NATO.” International law forbids territorial conquest. NATO’s mutual defense treaty should make the whole scenario impossible.
The Trump administration executed operations in Venezuela without Congressional authorization or advance notification, justifying it through stretching what the Constitution allows. That precedent now hangs over every discussion of the island like a storm cloud that hasn’t broken yet.
The question isn’t if attacking the Danes would be illegal. It would be—spectacularly, multiply, undeniably illegal. The question is if those legal prohibitions would stop it from happening.
Military Refusal of Illegal Orders
Military law explicitly requires soldiers to refuse illegal orders. You can’t claim you were following orders if the order was illegal. For senior officers, the implications of accepting an order to attack the Danes would be immediate and severe. The order would violate multiple laws and constitute a war crime.
When Democratic Congress members released a video reminding personnel of their right to refuse illegal orders, the Department of War announced it was investigating Senator Mark Kelly for his statements. While some reports indicated Secretary of Defense Pete Hegseth delivered a formal reprimand, the investigation’s exact status and outcome remain unclear. Hegseth argued that Kelly’s statements created confusion about which orders were lawful. Kelly criticized Hegseth’s actions as retaliation, while JAG officers discussed the legal implications of refusing orders that could constitute war crimes.
Military leaders increasingly teach that refusing clearly illegal orders isn’t mutiny. It’s following the law.
If the president issued a direct order to initiate operations against the island, senior Pentagon officials would face an immediate legal choice: comply and face potential prosecution for executing an unlawful order, or refuse.
The problem is this depends on individual officers making the right choice, not on automatic rules. A president could theoretically cycle through commanders until finding ones willing to follow illegal orders. Military law creates delays but not a complete block. And it only applies after the fact—to personnel who execute the order, not to the president who issues it.
The chain of command is the strongest check available within the executive branch. But it depends entirely on individual officers making the right choice under enormous pressure, possibly at the cost of their careers.
Congressional War Powers Limitations
The Constitution gives Congress the power to declare war. The War Powers Resolution says the president must tell Congress within 48 hours of committing armed forces and limits such operations to sixty days without Congressional authorization.
Senator Tim Kaine has already promised to “force a vote in the Senate about no U.S. military action in Greenland or Denmark,” predicting “overwhelming bipartisan support.” Representative Ted Lieu, an Air Force veteran, has publicly urged military personnel not to obey an order to invade Greenland, explaining it would violate both the Constitution and international law.
Congress can only act after military operations begin. A president can initiate hostilities and maintain them for sixty days without Congressional authorization. In this scenario, sixty days would likely be more than sufficient. The Danish armed forces are substantially smaller. Taking over Greenland might take only days or weeks.
By the time Congress convened, debated, and voted on a resolution restricting the operation, forces could already be in control. Congress would face a done deal: control established, troops in place, and the political calculus shifted dramatically toward accepting the accomplished fact rather than forcing a confrontation with U.S. forces already entrenched.
The Venezuela operation demonstrated exactly this dynamic. The Trump administration didn’t notify Congress in advance, citing concerns that notification would endanger the mission. By the time Congress could act, the operation was complete. A war powers resolution limiting Trump’s authority regarding Venezuela received bipartisan support in the Senate—all Democrats plus five Republicans voted for it. The resolution passed the procedural vote 52-47 and advanced to a final vote, demonstrating both some bipartisan willingness to constrain executive action and the difficulty of achieving sufficient support to fully restrict presidential authority even after the fact.
Historical precedent suggests that once personnel are engaged in combat, support for restraining the president tends to soften considerably. The political pressure facing a Congress attempting to compel withdrawal of forces from territory already under control would be immense, especially if the operation was framed as necessary for national security and Arctic defense against Russian and Chinese expansion.
Judicial Constraints and Enforcement Gaps
Courts have traditionally assumed that decisions involving war, foreign policy, and the armed forces fall within executive authority and should not be subject to judicial review. Courts have started to challenge this rule, but significant limitations would likely prevent courts from fully blocking a presidential decision in real time.
When the Trump administration tried to take control of California National Guard troops in Los Angeles without state consent, federal judge Charles R. Breyer blocked the deployment. In a similar case out of Chicago, the Supreme Court eventually ruled against the Trump administration—but only after considerable time had elapsed.
If a Congressional member or other appropriate plaintiff immediately sought an injunction, courts might eventually block the operation. But only after days or weeks of litigation during which operations would continue. Even if courts eventually ruled the operation unconstitutional, a substantial foothold might already be established.
The Supreme Court recently allowed the executive to keep acting on national security claims while courts make their decision. The Court recently made it harder for judges to stop nationwide executive actions, which constrains judicial mechanisms for stopping executive behavior. One judge warned this could let the executive act without real limits.
The most significant limitation is enforcement: even if a federal judge issued an order prohibiting attacks, what mechanism would enforce that order? No one has ever tried to punish a sitting president or officers for ignoring a court order. The feasibility is uncertain. Courts can issue orders, but they have no independent enforcement mechanism—they rely on executive compliance. If the executive ignores a court order, the court has no real power.
NATO’s Internal Contradiction
NATO’s Article 5 says an attack on one member is an attack on all. But Article 5 creates a major problem if a NATO member attacks another NATO member. It was designed for attacks from outside NATO, not from within.
By the exact wording of Article 5, an attack on Danish territory would count as an armed attack on one NATO member that obligates all members to treat it as an attack on themselves. However, the NATO treaty also explicitly states that NATO members can’t use Article 5 against each other because it goes against what NATO stands for. Article 1 says NATO members must solve disputes peacefully and to “refrain in their international relations from the threat or use of force.”
While Article 5 does not require unanimous agreement to invoke collective defense—the North Atlantic Council makes the determination, and each member then decides individually what action it deems necessary—the attacking nation would likely use its position to block or complicate any coordinated NATO response.
Europe’s militaries combined couldn’t defeat the U.S. military. British and French capabilities, while substantial, would be insufficient to challenge U.S. superiority.
NATO members would probably respond with sanctions and diplomacy, not force. NATO could demand U.S. withdrawal, break relations, or expel the U.S. from the alliance—though no one has ever tried to remove a member from NATO. NATO members could cancel agreements allowing U.S. military bases on their territory, forcing the armed forces to relocate out of Europe. Germany, Poland, Britain, and other major powers could coordinate to make clear that operations in Europe would face significant complications.
Denmark has already planned for this possibility. Denmark’s defense committee chair revealed that the country has orders from 1952 requiring Danish forces to immediately engage any invading army. Danish PM Frederiksen has stated: “If the United States chooses to attack another Nato country militarily, then everything stops.” Such a conflict would be one the Danes couldn’t win.
International Law Without Enforcement
The UN Charter’s Article 2(4) forbids countries from threatening or attacking each other. Breaking this rule theoretically means a country must stop, promise not to repeat it, and pay for damages.
In practice, international law is hard to enforce, especially against powerful countries. A powerful UN member can block any punishment against itself. The International Court could declare the U.S. broke the law, but the U.S. could simply ignore the ruling.
Europe would probably respond with economic sanctions. European officials are now exploring this possibility as a potential consequence. Blocking U.S. tech and finance companies would hurt the economy.
But all 27 EU members would have to agree to sanctions and would take weeks to implement—by which time the operation would be done. Europe trades with the U.S., so sanctions would hurt Europe too, making them reluctant to act.
The Enforcement Gap
The legal prohibitions on attacking a NATO ally are clear and numerous, yet nothing would definitely stop it from happening.
The strongest check is officers refusing to follow illegal orders. Career officers would face a genuine legal choice to refuse an order to attack a NATO ally without Congressional authorization. But this only works if individual officers make the right choice—it’s not automatic. The president could replace officers who refuse with ones who will obey.
Congress could stop this, but operations could be finished before Congress acts. The 60-day War Powers limit wouldn’t be enough—the U.S. could conquer Greenland in days or weeks. Even if Congress unanimously opposed it, voting would take days or weeks—by which time the armed forces would be in control.
Courts move too slowly to stop operations in real time. Courts might eventually block action, but only after days or weeks of delay, by which time the U.S. would already control the island. Courts depend on the executive obeying their orders—which the president could refuse to do.
NATO couldn’t respond militarily because the U.S. could block any NATO action. Other NATO members could only respond with sanctions and diplomacy, which would take time. International law forbids it, but there’s no way to enforce it against the U.S.
A president trying this would face opposition from Congress, courts, NATO, and the armed forces. But nothing would definitely prevent it. Officers would delay; Congress would spend days or weeks trying to stop it; courts would file lawsuits; NATO members would coordinate a response; and international law would be violated but not enforced.
The question isn’t whether there are ways to stop the president. It’s whether they would work fast enough and together to prevent the U.S. from taking over.
If institutions fail, the Constitution can’t stop a president from attacking an ally. Preventing war depends on the president choosing not to attack and Congress being willing to stop him—not on automatic rules. This scenario tests whether political and institutional constraints would work against an administration willing to break the law.
The Trump administration’s willingness to test constitutional boundaries—demonstrated through the Venezuela operation, attempts to federalize National Guard units without state consent, and public statements about acquiring Greenland—has forced a reckoning with questions that previously remained theoretical. The mechanisms designed to constrain executive power exist. Whether they would function effectively when challenged by a determined administration remains an open question. The answer may depend less on legal text than on the courage of individuals within institutions to enforce constraints that the law establishes but cannot, by itself, guarantee.
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