What Authority Does the President Have to Seize Foreign Territory?

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The United States has never faced this question in modern history: What legal authority does a sitting president have to seize foreign territory by force? President Donald Trump says the United States will acquire Greenland “one way or the other”—the easy way through negotiation or “the hard way” through force.

An administration is openly discussing options against Greenland—territory belonging to Denmark, a NATO member since the alliance’s founding in 1949—and dismissing legal objections as procedural obstacles rather than binding constraints.

Congressional War Powers Under the Constitution

The Constitution grants Congress, not the president, the sole power to declare war. The founders had fought a revolution against a monarch who could drag a nation into war on personal whim, and they designed a system specifically to prevent any single official from wielding that concentrated power.

A key Supreme Court ruling established that presidential authority reaches its lowest point when exercised contrary to Congressional will. Seizing foreign territory without Congressional authorization clearly violates this rule.

The Constitution also grants Congress control over how the country acquires new land. As constitutional scholar Andrew McCarthy notes, nowhere in the Constitution does the president possess authority to acquire territory through purchase or conquest.

Even Thomas Jefferson worked through Congress for the Louisiana Purchase. He negotiated the treaty with France, secured Senate ratification, and obtained Congressional appropriation of $15 million. Jefferson briefly worried whether the federal government possessed any authority to acquire territory at all—he considered seeking a constitutional amendment before concluding that a treaty plus Congressional funding was sufficient legal authority. The point that troubled him wasn’t whether the president could act alone. That question never came up because the answer was obvious.

The War Powers Resolution and Congressional Constraints

After Vietnam, Congress passed the War Powers Resolution in 1973, overriding the president’s veto. The statute requires a president who commits armed personnel without Congressional authorization to notify Congress within 48 hours and seek authorization within 60 days. If Congress neither authorizes the use of military action nor declares war within that window, the president must stop operations within 30 additional days.

For a seizure of Greenland, these constraints create a significant problem. The War Powers Act provides a limited pathway where a president could start action and then argue that subsequent Congressional debate constitutes authorization—but that pathway requires Congress to approve the action. Democratic and Republican Congressional members have already begun organizing against action on Greenland. Senator Tim Kaine and others have indicated that both chambers would unite to block any action directed at acquiring the territory.

Trump has characterized the War Powers Act as “unconstitutional” and asserted that presidents possess inherent authority as commander-in-chief to wage war without Congressional authorization. Legal experts and courts have rejected this idea. It conflicts with the Constitution’s text. Secretary of State Marco Rubio’s testimony to Congress acknowledged that the administration is considering options, yet attempted to characterize such action as a domestic law enforcement matter rather than an act of war—a characterization that contradicts the administration’s own public statements.

International Law Prohibits Territorial Conquest

The UN Charter, which America agreed to, prohibits countries from using military action to take territory. This provision represents one of the most fundamental norms in modern international law—the foundation upon which the entire post-World War II order rests.

The only times military action is allowed are in self-defense or with UN approval. Neither applies to Greenland. Greenland poses no armed threat, and any Security Council resolution authorizing action would be subject to U.S. veto, making the authorization mechanism theoretically available but politically absurd.

The UN Security Council cannot take action to stop U.S. operations because the U.S. can veto any such resolution. Instead, enforcement happens through diplomacy, pressure from allied countries, economic penalties, and potentially the International Court of Justice. The practical consequence is diplomatic isolation and erosion of U.S. credibility in invoking rules against other nations’ territorial expansionism.

NATO Treaty Obligations and Alliance Rupture

Greenland is territory of Denmark, a founding NATO member since 1949. The North Atlantic Treaty includes a collective defense provision that the U.S. invoked after 9/11 and cited to bind European allies to U.S.-directed operations in Afghanistan.

This creates a critical problem: Greenland is technically protected by NATO’s collective defense provisions, yet that protection depends entirely on the solidarity of all alliance members, with the U.S. holding veto power over its own enforcement. A U.S. attack on Danish personnel in Greenland would create unprecedented conflict between NATO members, forcing member states to choose between their obligation to Denmark and their security dependence on the U.S. The alliance would likely fracture entirely.

Historical Precedent for U.S. Territorial Acquisition

Every time the U.S. acquired new land, it did so through official legal processes that gave Congress the primary role, usually by having Congress approve a treaty and fund it.

The Louisiana Purchase of 1803 proceeded through treaty negotiation, Senate ratification, and Congressional appropriation of the $15 million purchase price. Alaska’s acquisition in 1867 followed the same pattern: Secretary of State William Seward negotiated the purchase from Russia, the Senate ratified the treaty, and Congress appropriated the $7.2 million purchase price.

The Virgin Islands purchase from Denmark in 1917 is particularly relevant. When the U.S. previously acquired Danish territory, it did so through formal treaty negotiation, Senate ratification, and Congressional appropriation of $25 million. Trump’s assertion of presidential authority through military action represents a radical departure from that precedent.

Military and Legal Obstacles to Seizure

If a president ordered personnel to seize Greenland, substantial operational and legal constraints would apply. An operation against an allied nation whose citizens have expressed no desire to become part of the U.S. would constitute use of military action against a population not engaged in hostilities.

Military law requires soldiers to follow lawful orders while simultaneously prohibiting obedience to unlawful orders. Military commanders can and must refuse orders they believe are illegal. An officer receiving orders to invade and occupy Greenland would face a genuine legal dilemma: the order purports to emanate from the commander-in-chief, yet no Congressional authorization exists for hostilities against Denmark, no armed attack has occurred, and the order would violate existing NATO treaty obligations. Lawyers advising commanders would likely say the order is illegal because Congress didn’t approve it.

Operationally, Greenland’s population of approximately 57,000 people possesses no military capability and would offer minimal conventional resistance. Yet occupation and administration would require sustained presence, administrative apparatus, and resources directed at managing the political and social consequences of forcing them to become part of the U.S. against their will and against the wishes of the Danish government.

Danish personnel stationed in Greenland would face the legal duty to resist occupation of Danish territory. Rasmus Jarlov, who leads Denmark’s defense committee, said that if the U.S. attempted takeover of Greenland, Danish soldiers would be required “to fight back against the Americans.” That would mean NATO allies fighting each other—precisely the catastrophic rupture that NATO’s founders sought to prevent.

Congressional Options to Prevent Action

Congress has several ways to stop this. Congress could proactively pass legislation prohibiting use of federal funds for action against Greenland, Denmark, or NATO allies without explicit Congressional authorization.

Democratic Representative Seth Magaziner introduced legislation in March 2025, the “No Invading Allies Act,” specifically designed to prevent Trump from action against Canada, Greenland, or Panama without Congressional vote. This legislation would stop the military from spending money on an invasion of these territories without explicit Congressional approval. While Trump could theoretically veto such legislation, Congress could pass it without the president’s approval if enough members vote for it. Both Democrats and Republicans have indicated they would vote against such action.

Congress could also use the War Powers Resolution: if a president initiated operations against Greenland, either the House or Senate could refuse to fund the operation within 30 days. Senator Tim Kaine has already indicated he would pursue such action.

Escalation From Purchase to Military Force

Trump’s statements regarding Greenland have escalated substantially from initial discussion of purchase to explicit threat language. Initially, the administration framed acquisition as a potential purchase, analogous to Alaska negotiations or the Louisiana Purchase. Yet as Denmark and Greenland repeatedly rejected such discussions, Trump’s language shifted.

By January 2026, he was stating that the U.S. “will” acquire Greenland “one way or the other,” that he would do it “the easy way” through deal-making or “the hard way” through military action, and that he felt territorial “ownership” was “psychologically needed for success.” This rhetorical shift reflects an assertion of presidential authority: when negotiation fails, the president claims to possess authority to resort to seizure.

Trump’s Deputy Chief of Staff for Policy Stephen Miller, when asked in a CNN interview whether the U.S. would seek Greenland by military action, responded with rhetorical questions challenging Denmark’s legal basis for controlling Greenland and asserting that “the United States is the power of NATO” and that “obviously Greenland should be part of the United States.” This framing attempts to shift the legal question from whether the president possesses authority to use military action to whether Denmark possesses legitimate authority over Greenland (it does, under the Self-Government Act of 2009).

This pattern mirrors the administration’s justification for action in Venezuela. Secretary of State Rubio and other officials characterized the January 2026 seizure of Venezuelan President Nicolás Maduro as a “law enforcement” operation despite employing military action, helicopter insertions, armed combat, and the abduction of a foreign head of state. The rhetorical strategy involves minimizing action as something less than “war” in order to argue that the president possesses executive authority to conduct it without Congressional authorization. Courts and constitutional scholars have repeatedly rejected such characterizations. Action that involves armed personnel, weapons deployment, and potential for combat constitutes military action regardless of how officials characterize it.

International Consequences and Global Precedent

A U.S. seizure of Greenland would establish a dangerous precedent and would likely reshape other nations’ assumptions about commitment to rules and treaty obligations. If the U.S., a nuclear-armed superpower and founding member of the UN and NATO, could openly violate territorial integrity prohibitions and undertake seizure of allied territory, then the legal frameworks constraining territorial expansion would lack credibility.

Russia’s seizure of Crimea in 2014 proceeded through claims of protecting ethnic Russians and invoked historical, cultural, and security justifications parallel to Trump’s Arctic security claims. The response included sanctions and diplomatic isolation, yet Russia retained control of the territory. A U.S. seizure of Greenland would render criticism of Russian actions hypocritical and would encourage other nations to reassess whether territorial claims backed by military action might succeed despite prohibitions.

China, which maintains territorial disputes with multiple neighbors regarding islands and maritime zones in the South China Sea, would likely draw the conclusion that claims regarding rules apply selectively to nations the U.S. opposes rather than universally to all nations. India, Brazil, and other nations with territorial disputes or expansionist ambitions would similarly observe that constraints on territorial acquisition and military action appear to apply selectively based on power and geopolitical interests.

For the NATO alliance specifically, a U.S. seizure of Greenland would likely accelerate European efforts toward the ability to defend themselves without relying on America and reduce European dependence on security guarantees. A U.S. action threatening NATO’s territorial integrity would likely shift European opinion decisively toward independence, prompting investment in independent European capabilities and potentially accelerating the development of EU collective defense mechanisms as alternatives to NATO. The paradox is that pursuit of Greenland through military action would likely undermine dominance precisely through fracturing the alliance upon which that dominance ultimately rests.

Whether Constitutional Limits Can Prevent Presidential Action

The fundamental question posed by Trump’s Greenland rhetoric is whether constitutional and statutory constraints on presidential war powers work to prevent a president from undertaking operations the president believes necessary. The domestic constitutional framework—giving war declaration authority to Congress, requiring Congressional authorization for action, constraining presidential executive power through statutory limits like the War Powers Resolution—assumes that these legal prohibitions will deter a president from undertaking unlawful action. But if a president is willing to openly violate these constraints and claim inherent constitutional authority to do so, what mechanism exists to stop them?

The practical answer involves multiple potential checks. Lawyers and command structure could decline to execute orders a legal advisor concludes are unlawful, creating direct operational obstacles to action. Congress could pass legislation explicitly prohibiting action, creating a funding cutoff that would prevent operations from continuing. If operations proceeded despite Congressional prohibition, Congress could initiate impeachment proceedings. Command could potentially be compelled to comply with Congressional directive through court orders forcing the military to stop, though federal courts have traditionally been reluctant to intervene in ongoing operations.

Refusal to execute orders would require legal advisors to conclude that orders were unlawful—a judgment made in real time under pressure and potentially subject to political pressure from civilian leadership. Congressional action would require mobilizing sufficient votes to pass legislation against a president’s opposition, a process that requires time during which operations could be initiated and partially completed. Impeachment is a lengthy political process that would not stop ongoing operations. Federal court intervention in operations remains a legally complicated area where courts are hesitant to act.

The practical consequence is that a president willing to openly flout constitutional constraints possesses the capability to initiate and partially complete operations against Greenland before institutional constraints could be brought to bear. As constitutional scholar Andrew McCarthy notes, the War Powers Act’s 60-day window could theoretically permit seizure of Greenland’s small population and undefended territory before Congress could intervene.

The Legal Framework and Its Enforcement

The question “What authority does the president have to seize foreign territory?” has a clear legal answer: none, beyond the authority Congress explicitly grants through declaration of war or specific authorization for military action. The Constitution gives war powers to Congress. Rules prohibit territorial conquest absent UN Security Council authorization. NATO treaty obligations create legal entanglements for action against allies. Historical precedent demonstrates that even exceptionally assertive presidents have worked through legal mechanisms requiring legislative involvement.

Yet Trump’s claim that he can take Greenland by military action if he wants demonstrates that the gap between legal prohibition and presidential assertion of power has narrowed dangerously. The president claims to possess inherent constitutional authority as commander-in-chief to undertake operations he believes necessary without Congressional authorization—but the Constitution, Supreme Court, and Congress all say he cannot.

The operation in Venezuela in January 2026 showed this administration will use force against another nation’s head of state without Congressional authorization, even though it violated the War Powers law and UN rules on territorial intervention. Yet neither has stopped ongoing involvement in Venezuelan governance. The administration ignored rules and Congress’s objections, suggesting that legal constraints alone may not prevent action against Greenland if the administration determines such action necessary.

Congress possesses overwhelming legal authority to prevent seizure through appropriations power, war powers resolutions, and explicit legislative prohibition. Federal courts can order the military to stop illegal operations and to hold officials accountable for constitutional violations. Legal advisors possess authority to advise commanders that presidential orders are unlawful. But each one only works if Congress, courts, or leaders are willing to stand up against a sitting president’s claim of emergency power and commander-in-chief authority.

The Greenland situation is a test of whether constitutional limits work when a president claims powers the law doesn’t give him and ignores legal objections. History shows constitutional limits work best when Congress is willing to use its power. The coming months will show whether constitutional limits can stop the president willing to explicitly challenge them.

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