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The short answer, under international law, is no: The use of coercion to acquire another country’s territory is forbidden. The question arises since President Donald Trump threatened on Saturday to impose new tariffs on several European countries unless Denmark sells Greenland to the United States. He said that the United States must purchase Greenland to safeguard its national security and ensure “world peace.” As a matter of domestic law, however, the US president has no authority to apply tariffs for this purpose.
The president does not have the authority to acquire territory on his own. He can negotiate a treaty for the United States to acquire a territory and ask the Senate to ratify it and the House of Representatives to initiate a bill to pay for it (if any payment is needed). Every recognized US territorial acquisition involved Congress: the Louisiana Purchase in 1803, by treaty ratified by the Senate and with House Appropriations; Florida in 1819 by treaty; the annexation of Texas in 1845 by joint resolution of Congress; the Mexican Cession in 1848 by the Treaty of Guadalupe Hidalgo; the Alaska Purchase in 1867 by treaty ratified by the Senate; the annexation of Hawaii in 1898 by joint resolution of Congress; and Puerto Rico, Guam, and the Philippines, by the Treaty of Paris in 1898.
A recent Reuters/Ipsos poll, conducted before Trump’s announcement Saturday, found just 17 percent of Americans approved of his efforts to acquire Greenland. Some 47 percent of respondents to the poll disapproved of US efforts to acquire Greenland, while 35 percent said they were unsure.
Presidents can order US military occupation of territory temporarily under war powers, as in Germany and Japan after WWII, and Iraq in 2003. But occupation does not equal sovereignty. Territory is not “acquired” unless Congress acts. The foundational legal case addressing this point is American Insurance Co. v. Canter (1828), under Supreme Court Chief Justice John Marshall.
The current situation also raises the question: Has Congress granted the president legal authority to impose tariffs on North Atlantic Treaty Organization (NATO) allies that refuse to agree to recognize Trump’s threatened annexation of Greenland? The answer is “no.”
Trump’s mention of US national security as a justification for his threatened tariffs over Greenland suggests he might again invoke the International Emergency Economic Powers Act (IEEPA) as a source of legal authority, as he did with tariffs against most countries he ordered in 2025. The Supreme Court has alerted the press that it could rule any day now on legal challenges to those tariffs. My guess is that the court will rule against the blanket “reciprocal tariffs” that Trump announced on April 2, 2025, but may allow more selective tariffs that might be more closely related to an emergency. Several IEEPA tariffs cases have been stayed awaiting the court’s decision. The IEEPA cases would not necessarily resolve whether Trump could use IEEPA in the case of Greenland because no such tariffs have been imposed yet.
A lot has happened involving IEEPA since I wrote about a possible US acquisition of Greenland a year ago—most prominently, the challenges on which the Supreme Court will rule. We know more about IEEPA. (The Amicus brief that I co-wrote can be read here.) For example: We know that this kind of statute has never been used before to impose US tariffs by presidential decree. Trump administration lawyers claimed during litigation in the lower courts last year that President Richard Nixon had invoked emergency authority to impose tariffs in the 1970s. He didn’t. We know that the Congress did not view IEEPA as a general tariff statute but as an economic sanctions statute.
Courts have never overturned presidential declarations of emergency under IEEPA. Congress entrusted the president acting in an emergency that he had identified. But a Supreme Court ruling on using IEEPA tariffs to coerce allies over the United States acquiring Greenland might overcome previous judicial restraint. Nearly all instances of IEEPA economic pressure (none including using tariffs) have been against enemies. There are exceptions, for example, such as using the IEEPA to press foreign banks in neutral and allied countries to align their practices with US sanctions policy. Those instances are not relevant here, as the US government would be pressing foreign governments not to oppose the forced US acquisition of Greenland contrary to international law.
It would be reasonable for the Supreme Court to cast aside its traditional deference to the executive in matters involving foreign affairs and declare that the Congress needed to be directly involved in an act of economic aggression. The court should not countenance the manufacture of an emergency as a pretext to invoke IEEPA. Congress, when passing IEEPA, never contemplated a use so unbounded by all precedent and past US policy. In effect, the court could well hold that IEEPA was designed as a defensive measure and not a means to attack allied and neutral countries’ commerce. Such an attack would require congressional approval.
To stay out of foreign wars (at that time between Great Britain and France), both presidents George Washington and Thomas Jefferson worked with Congress to apply embargoes to US trade with those countries to avoid becoming entangled in foreign conflicts. Congress allowed both presidents to administer the embargoes. Their purpose was to avoid confrontation, not provoke it. The founders all would have been astounded if the president thought that he could under any pre-existing delegated or constitutional power on his own engage in economic warfare against any other country. Furthermore, they knew that the power to tax Americans for this purpose through a tariff would require very explicit and special authority from Congress. They had just fought a long and hard war with Great Britain to avoid paying for wars unless their own elected representatives first authorized the taxes.
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