By: Yedida Bentolila
Greenland has emerged as a modern geopolitical dilemma given recent statements made by President Donald Trump. He has expressed interest in acquiring Greenland and has renewed longstanding questions about the legal and political authority of the United States to annex foreign territory. Although the United States has not added permanent territory through purchase or cession in more than 70 years, territorial expansion historically occurred through formal agreements that were widely understood at the time to be lawful under prevailing international norms. Greenland, however, presents a different dynamic, since its location is of strategic interest for maritime routes, Arctic exploration, and defense planning, as well as other areas of interest for the United States.
Since its founding in 1776, the United States has dramatically transformed its geographic footprint in the nineteenth and early twentieth centuries. What began as a nation covering approximately 865,000 square miles has grown to more than 3.5 million square miles, a development accomplished through a series of purchases, cessions, and treaty-based transfers between sovereign states.
The constitutional framework governing this expansion relied on implied federal powers rather than an explicit annexation clause. Treaties were negotiated by the executive branch and approved by a two-thirds vote of the Senate under Article II, while Congress exercised authority over newly acquired lands pursuant to Article IV, which grants it power to regulate U.S. territories.
The most transformative acquisition under this model was the Louisiana Purchase of 1803, through which the United States acquired nearly 828,000 square miles of land from France for $15 million, effectively doubling its size. Despite President Thomas Jefferson’s concern that the Constitution did not expressly authorize the purchase of foreign territory, the acquisition was completed by treaty and later affirmed as constitutional by the Supreme Court. Similar transactions followed throughout the nineteenth century. Florida was acquired from Spain in 1819 through treaty, Alaska was purchased from Russia in 1867 for $7.2 million, and large portions of the Southwest were transferred from Mexico under the Treaty of Guadalupe Hidalgo and the Gadsden Purchase. In each case, territorial expansion was effected through negotiations between recognized sovereign states and formalized by treaty.
In addition to treaty-based acquisitions, the United States asserted control over numerous small islands in the Pacific and Caribbean through occupation under the Guano Islands Act of 1856. Following World War II, the United States acquired the Northern Mariana Islands, Caroline Islands, and Marshall Islands under international trusteeship arrangements, marking the last major instance of U.S. territorial acquisition. These territories, along with Puerto Rico, Guam, and the U.S. Virgin Islands, remain under U.S. sovereignty but lack full political integration as states. Taken together, this history shows that U.S. territorial expansion rested on two premises: consent by the transferring sovereign and an international legal order that tolerated permanent transfer of territory. While past expansion occurred in an era when territorial transfers between states were common and widely accepted, modern international systems place greater emphasis on the political rights of local populations and self-determination. These constraints differentiate modern proposals, such as Greenland, from historical precedent.
Greenland is an autonomous territory within the Kingdom of Denmark, with its own parliament and substantial self-governing authority, while Denmark retains responsibility for defense and foreign affairs. Neither Denmark nor Greenland has expressed any willingness to transfer sovereignty, and Greenland itself has not sought annexation or independent statehood, circumstances that historically accompanied U.S. territorial expansion.
Unlike earlier acquisitions, the current proposal does not arise from armed conflict, post-war settlement, or decolonization. President Trump has publicly called for negotiations to discuss a potential acquisition of Greenland, initially framing the issue as a diplomatic matter and citing national security reasons. More recently, the administration has escalated its rhetoric by threatening economic measures, including tariffs, against Denmark and other European states.
Importantly, the United States already maintains a formal legal agreement governing its security interests in Greenland. In 1951, during the Cold War, the United States and Denmark entered into the Defense of Greenland Agreement, which grants the United States extensive military presence and operational rights on the island while preserving Danish sovereignty. That agreement remains in force today. Moreover, both the United States and Denmark are NATO allies, and Greenland’s security has long been understood as part of NATO’s collective defense structure. This existing defense arrangement weakens the historical justification for annexation by demonstrating that U.S. security objectives in Greenland have already been achieved without a transfer of sovereignty.
U.S. interest in Greenland is primarily rooted in strategic and security considerations. Greenland occupies a critical geographic position in the Arctic, located between North America and Europe and adjacent to increasingly navigable Arctic shipping routes. The Arctic has assumed greater importance for military mobility, early warning systems, and defense planning, and Greenland in turn plays a major role. In addition, Greenland is believed to contain significant natural resources, including rare earth minerals that are increasingly relevant to modern defense systems. These factors have intensified U.S. interest in maintaining secure access and influence in the region.
Historically, the United States has acquired territory through treaties approved by the Senate or, in limited cases, through joint resolution of Congress. Applying these mechanisms to Greenland would require Denmark’s consent to transfer sovereignty, followed by approval through the U.S. constitutional process. Under modern international norms, any lawful transfer would also require the consent of Greenland’s population.
Greenland differs materially in ways that undermine reliance on historical annexation precedent. Earlier U.S. territorial expansions typically occurred through purchase from a willing sovereign or following armed conflict that altered territorial control; neither condition is present here. Denmark is a stable democratic state and a NATO ally, and Greenland exercises autonomous governance, reflecting contemporary norms of self-determination rather than classical colonial dependency. Moreover, U.S. security interests in Greenland have already been addressed through the 1951 Defense of Greenland Agreement, which provides extensive military access without transferring sovereignty.
In a post-World War II legal framework, U.S. security objectives are generally pursued through alliances and negotiated access arrangements rather than territorial acquisition. This framework diminishes any claim that annexation is necessary to achieve U.S. security objectives. Greenland therefore exists in a fundamentally different legal and political environment. While constitutional mechanisms for territorial acquisition remain formally available, modern international norms, NATO alliance commitments, and existing defense agreements substantially constrain their application. The Greenland proposal therefore highlights the practical and legal limits of applying nineteenth-century expansion models to contemporary geopolitical contexts.
