What Authority Does the President Have to Seize Foreign Territory?

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Under the U.S. Constitution and Supreme Court rulings, a president cannot annex foreign land alone. Extending U.S. borders requires a treaty made with the Senate’s advice and consent or an Act of Congress under Fleming v. Page (1850), while Article IV, Section 3, Clause 2 gives Congress authority over federal territory.

As Commander-in-Chief, a president can direct troops during an authorized war, but wartime military occupation of foreign territory does not make that land part of the United States. Outside an armed attack on the nation, the War Powers Resolution of 1973 restricts sending troops into combat without congressional approval, and the United Nations Charter prohibits using force against another state’s territorial integrity.

Those rules took center stage in 2025 and 2026 after the Trump administration sought U.S. control of Greenland and military access near the Panama Canal. Instead of border changes, talks produced a U.S.-Panama defense accord that Panamanian officials asserted did not cede sovereignty and an announced U.S.-Denmark security deal on Greenland.

How the Constitution Divides Territory Power

Under the Property Clause of Article IV, Section 3, Clause 2, “The Congress shall have Power to dispose of and make all needful Rules and Regulations respecting the Territory or other Property belonging to the United States.” That clause governs federal land once acquired. In American Insurance Co. v. Canter (1828), Chief Justice John Marshall held that because the Constitution gives the Union the war and treaty powers, it also has the power to acquire land by conquest or treaty.

Writing for the Court in American Insurance Co. v. Canter, Chief Justice Marshall separated wartime control from annexation. If a nation is not entirely subdued, holding conquered land during war is “a mere military occupation, until its fate shall be determined at the treaty of peace.” When land is ceded by treaty, the gain is confirmed and the land is annexed.

During the Mexican-American War, as the Constitution Annotated explains, U.S. military forces governed the occupied Mexican port of Tampico. In that tariff case, Fleming v. Page (1850), an importer argued that goods shipped from Tampico were exempt from foreign import duties. The Supreme Court rejected that tariff claim because military rule did not make the port part of the United States.

Writing for the Court in that 1850 Tampico decision, Chief Justice Roger Taney wrote that the President’s wartime “duty and his power are purely military.” Although the President may invade a hostile country during war, “his conquests do not enlarge the boundaries of this Union,” which “can be done only by the treaty-making power or the legislative authority.”

To buy the Danish West Indies, the United States used that treaty process. Under the 1916 Convention for the Cession of the Danish West Indies, proclaimed on January 25, 1917 after the Senate advised and consented, the United States paid Denmark “twenty-five million dollars in gold coin.” Signing on August 4, 1916, Secretary of State Robert Lansing formally recognized Danish authority in Greenland. He declared that the United States “will not object to the Danish Government extending their political and economic interests to the whole of Greenland,” backing Danish rule long before the modern dispute.

One law complicates the rule that a president cannot add land alone. The Guano Islands Act of 1856 (48 U.S.C. § 1411) says a U.S. citizen who discovers guano on an island, rock, or key may take peaceable possession. If that island is “not within the lawful jurisdiction of any other government, and not occupied by the citizens of any other government,” it “may, at the discretion of the President, be considered as appertaining to the United States.” That rule comes from Congress and excludes Greenland, a self-governing part of Denmark.

Why Greenland and Panama Sparked Debate

As a CRS report on Western Hemisphere policy explained, the Trump administration argued in 2025 and 2026 that U.S. control of Greenland is vital to national security. The administration cited the need “both to counter China and Russia in the Arctic and to execute the President’s proposed Golden Dome missile defense plan.” In June 2025, U.S. Northern Command (NORTHCOM) moved Greenland from U.S. European Command into NORTHCOM’s area of responsibility.

In January 2026, the White House told the BBC that acquiring Greenland was a “national security priority.” The White House added: “The president and his team are discussing a range of options to pursue this important foreign policy goal, and of course, utilising the US military is always an option at the commander-in-chief’s disposal.”

Administration officials and allies in Congress backed that position. White House Deputy Chief of Staff Stephen Miller said in January 2026 that it was “the formal position of the US government that Greenland should be part of the US.” Asked on CNN whether the administration would rule out force to annex the island, Miller replied: “Nobody’s going to fight the US over the future of Greenland.” Sen. Eric Schmitt of Missouri told the BBC: “My hope is that Europe would understand that a strong America is good – it’s good for Western civilisation.”

European and Greenlandic leaders rejected any takeover or threat of force. In a January 2026 joint statement, the leaders of the UK, France, Germany, Italy, Poland, Spain, and Denmark declared that “Greenland belongs to its people, and only Denmark and Greenland can decide on matters concerning their relations.”

Those European leaders urged “upholding the principles of the UN Charter, including sovereignty, territorial integrity and the inviolability of borders.” Greenland Prime Minister Jens-Frederik Nielsen said talks require respect for the fact that “Greenland’s status is rooted in international law and the principle of territorial integrity.”

As a CRS report noted, over 40% of U.S. container traffic reportedly transits the Panama Canal each year, and the canal passed to Panama in 1999 under treaties approved by the Senate in 1978. In April 2025, Defense Secretary Pete Hegseth signed a defense memorandum to reestablish “rotational and joint presence” near the canal. Panamanian officials asserted “that the agreement does not cede Panamanian sovereignty and the country will not accept foreign military bases on its territory, in accordance with the Panama Canal treaties.”

In Greenland, where the United States runs Pituffik Space Base under the 1951 U.S.-Danish defense agreement, the CRS report noted that President Trump said in January 2026 he “won’t use force” in Greenland. U.S., Danish, and Greenlandic officials then opened talks through a high-level working group.

In the September 2026 Greenland security agreement, leaders said the deal was expected to be signed the next week, with no text yet released. President Trump said Denmark would grant the United States “permanent control over security, and all other needs” and bar adversaries from keeping forces there. Danish Prime Minister Mette Frederiksen and Greenland Prime Minister Jens-Frederik Nielsen said the deal “strengthens our common security in the Arctic and the North Atlantic area” while recognizing “the sovereignty and territorial integrity of the Kingdom and the right of the Greenlandic people to self-determination.”

How War Powers Limit Unilateral Force

Under Article I, Section 8, Clause 11 of the Constitution, Congress holds the power “To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water.” In Youngstown Sheet & Tube Co. v. Sawyer (1952), the Supreme Court struck down President Harry Truman’s Korean War seizure of domestic steel mills without congressional approval.

Concurring in Youngstown Sheet & Tube Co. v. Sawyer, Justice Robert Jackson set out the test for judging presidential power against Congress’s power. Justice Jackson wrote: “When the President takes measures incompatible with the expressed or implied will of Congress, his power is at its lowest ebb, for then he can rely only upon his own constitutional powers minus any constitutional powers of Congress over the matter.”

Congress added statutory limits in the War Powers Resolution of 1973. Under 50 U.S.C. § 1541(c), the President’s powers to send U.S. Armed Forces into hostilities apply in only three cases. Those are “(1) a declaration of war, (2) specific statutory authorization, or (3) a national emergency created by attack upon the United States, its territories or possessions, or its armed forces.”

Under 50 U.S.C. § 1543(a), when U.S. forces enter hostilities under clause (1) or enter foreign territory, airspace, or waters equipped for combat under clause (2) without a war declaration, the President must report in writing within 48 hours. Under clause (2), that combat-equipped deployment report is not triggered by “deployments which relate solely to supply, replacement, repair, or training of such forces.”

Under 50 U.S.C. § 1544(b), within sixty calendar days after a hostilities report under section 1543(a)(1) of the law, the President must end that use of forces unless Congress declares war, enacts a specific authorization, or extends the time. The sixty-day limit can be extended for up to thirty days if the President certifies in writing that “unavoidable military necessity respecting the safety of United States Armed Forces” requires it for a prompt removal.

Within the chain of command, Uniform Code of Military Justice Article 92 (10 U.S.C. § 892) punishes service members who fail to obey “any other lawful order,” conditioning the duty to obey on the order being lawful. To make any unauthorized invasion order unlawful, lawmakers in the 119th Congress introduced H.R. 1936 to “restrict the President from ordering or utilizing the United States Armed Forces” to invade Canada, Panama, or Greenland, while H.R. 361 would authorize purchase talks.

What UN, NATO, and Danish Law Require

Under Article 2(3) and Article 2(4) of the United Nations Charter, UN members must settle disputes “by peaceful means” and “refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state.” Article 51 keeps “the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations.”

Under Article 23(1) and Article 27(3) of the United Nations Charter, the United States is a permanent Security Council member and can veto Council enforcement actions against itself because substantive votes require nine yes votes “including the concurring votes of the permanent members.”

Under Article 1 of the North Atlantic Treaty, which took effect on 24 August 1949, NATO allies pledge “to refrain in their international relations from the threat or use of force in any manner inconsistent with the purposes of the United Nations.” Under Article 5, “an armed attack against one or more of them in Europe or North America shall be considered an attack against them all.” Article 6 defines that area to include “the Islands under the jurisdiction of any of the Parties in the North Atlantic area north of the Tropic of Cancer.”

As the Prime Minister’s Office of Denmark explains, the Act on Greenland Self-Government entered into force on 21 June 2009 and replaced the 1979 Home Rule Arrangement. In that office’s summary, the Act’s preamble recognizes that the people of Greenland are a people with a right to self-determination under international law.

Under the Act on Greenland Self-Government, if Greenlanders choose independence, talks begin between the Danish Government and Naalakkersuisut (the Greenland Government). Any deal requires the consent of Inatsisartut (the Greenland Parliament), backing “by a referendum in Greenland,” and the consent of the Folketing (the Danish Parliament) under Section 19 of the Danish Constitution.