Would It Be Legal for a U.S. President to Use the Military to Seize Greenland from Denmark?

A U.S. Constitutional and International Law Analysis

01/11/26

From time to time, public discussion resurfaces around whether the United States could acquire Greenland, an autonomous territory of the Kingdom of Denmark, through coercive means—including military force. While such scenarios are often discussed rhetorically, the legal framework governing them is well established.

This article examines whether a U.S. president—Donald Trump or any other—could lawfully deploy the United States military to seize control of Greenland by force, and explains the U.S. constitutional, statutory, and international law violations that would arise from such an action.

U.S. Constitutional Law: Limits on Presidential War Powers

Congress, Not the President, Declares War

Under Article I, Section 8 of the U.S. Constitution, Congress alone has the power to:

·       Declare war

·       Authorize military hostilities

The President’s authority as Commander in Chief does not include unilateral power to initiate war against a sovereign nation.

A military seizure of Greenland would constitute a war of choice, not a defensive action, and would require explicit congressional authorization.

Absent such authorization, the action would violate the Constitution.

The War Powers Resolution

The War Powers Resolution of 1973 further restricts presidential use of military force by requiring:

·       Congressional authorization within 60 days

·       Immediate reporting to Congress

·       Withdrawal of forces absent approval

A full-scale invasion or occupation of Greenland could not plausibly be justified as a short-term defensive deployment. It would exceed any recognized emergency authority under the statute.

Domestic Criminal and Civil Liability Concerns

While presidents enjoy broad immunity for official acts, military officers and civilian officials involved in executing unlawful orders may face:

·       Court-martial liability under the Uniform Code of Military Justice

·       Potential violations of federal statutes

·       International legal exposure under universal jurisdiction principles

The doctrine that “following orders” is not a defense to unlawful conduct is deeply embedded in both U.S. and international law.

NATO Treaty Obligations

Denmark is a member of NATO, and Greenland falls under Denmark’s sovereignty.

Under Article 5 of the North Atlantic Treaty, an armed attack against one member is considered an attack against all.

A U.S. military attack on Greenland would:

·       Violate NATO treaty obligations

·       Trigger a constitutional and diplomatic crisis

·       Undermine collective defense commitments

Such an act would place the United States in breach of its own defense alliances.

Greenland’s Legal Status Under International Law

Greenland is not terra nullius, nor is it an unclaimed territory.

·       Greenland is an autonomous territory within the Kingdom of Denmark

·       Denmark retains responsibility for foreign affairs, defense, and sovereignty

·       Denmark is a NATO member state and a recognized sovereign under international law

Any attempt to seize Greenland by military force would therefore constitute the use of force against the territorial integrity of another sovereign nation.

International Law: Prohibition on the Use of Force

Article 2(4) of the United Nations Charter

Under Article 2(4) of the U.N. Charter, all member states—including the United States—are prohibited from:

“the threat or use of force against the territorial integrity or political independence of any state.”

A military seizure of Greenland would be a textbook violation of this prohibition.

No Applicable Exception Applies

There are only two recognized exceptions to the prohibition on the use of force:

1.     Self-defense under Article 51

2.     Security Council authorization

Neither would apply.

·       Denmark has not attacked the United States

·       Greenland poses no imminent armed threat

·       No plausible claim of anticipatory self-defense exists

·       The U.N. Security Council would not authorize aggression against a NATO ally

As a matter of international law, a forcible seizure of Greenland would be illegal aggression.

Crime of Aggression Under International Law

Customary international law—and the Rome Statute of the International Criminal Court (ICC)—defines the crime of aggression as:

the planning, preparation, initiation, or execution of an act of aggression by a state against the sovereignty of another state.

Although the United States is not a party to the ICC, the prohibition against aggression is widely regarded as customary international law, binding on all states.

A military invasion of Greenland would meet every recognized element of aggression under international law.

No Legal Mechanism for Territorial Acquisition by Force

Historically, international law once tolerated conquest. That doctrine was abandoned after World War II.

Today:

·       Territory cannot be lawfully acquired by force

·       Military occupation does not confer sovereignty

·       Annexation through coercion is legally void

Any attempt to seize Greenland militarily would be legally invalid, even if temporarily successful.

Could Greenland Be Acquired Legally?

The only lawful mechanisms for acquisition would be:

·       Voluntary transfer by Denmark

·       A lawful treaty approved by the U.S. Senate

·       A process consistent with international self-determination principles

Military force is not one of them.

Conclusion: A Clear Violation of U.S. and International Law

A U.S. president does not have legal authority to deploy the military to seize Greenland from Denmark.

Such an action would violate:

·       The U.N. Charter

·       Customary international law prohibiting aggression

·       U.S. constitutional limits on war powers

·       The War Powers Resolution

·       NATO treaty obligations

·       Fundamental principles of sovereignty and territorial integrity

From both a constitutional law and international law perspective, the answer is unequivocal:a military seizure of Greenland would be illegal.

Why This Legal Analysis Matters

Questions like these illustrate how constitutional law, international law, and executive power intersect—often with consequences extending far beyond politics.

At Biazzo Law, PLLC, we analyze legal questions through the lens of constitutional structure, appellate review, and rule-of-law principles, including matters involving federal authority and international legal obligations.