What legal and diplomatic constraints block a U.S. attempt to annex or occupy territory like Greenland?

1. Legal prohibition on acquisition of territory by force: the core barrier

Modern international law flatly rejects annexation through the threat or use of force: Article 2 of the UN Charter and later declarations establish that territorial conquest is unlawful, and contemporary scholarship and institutional guidance treat unilateral annexation as illegal ^{[1] [6] [7]}.

2. Peremptory norms, self‑determination and criminalization of aggression

Annexation implicates jus cogens norms—most notably the right of peoples to self‑determination—and can amount to an act of aggression punishable under instruments like the Rome Statute; international human rights bodies call annexation a “flagrant violation” of the UN Charter ^{[5] [3] [4]}.

3. Non‑recognition and the diplomatic consequences of illegality

Even a successful on‑the‑ground seizure would likely remain unlawful and unrecognized: states routinely refuse to acknowledge territorial gains made by force, applying doctrines like Stimson non‑recognition, and international practice shows unlawful annexations attract collective diplomatic and legal countermeasures ^{[4] [1] [2]}.

4. The Security Council, sanctions and the problem of enforcement

The UN Security Council is a primary forum for responding to annexation, and past crises show it can authorize diplomatic, economic or even military measures; while enforcement is politically constrained, Security Council engagement would shape global reactions and legitimacy ^{[2] [8]}.

5. Recognition, acquiescence and the limited path to de facto change

Scholars note a narrow, contested pathway whereby prolonged factual recognition can erode illegality into acceptance, but this is exceptional, slow, and politically fraught — it does not erase the initial illegality or the practical diplomatic costs ^{[8] [7]}.

6. U.S. domestic law does not negate international obligations

Some commentators observe that domestic mechanisms (e.g., an act of Congress) might attempt to incorporate foreign territory into U.S. constitutional structures, yet such internal steps cannot negate international prohibitions or prevent other states from treating the act as unlawful and responding accordingly ^{[9] [6]}. Reporting notes that while the U.S. Constitution does not explicitly forbid territorial acquisition, domestic approval would be “abhorrent” to many and would not immunize the U.S. from international law consequences ^[9].

7. Strategic and reputational costs beyond courts and charters

The diplomatic fallout would extend to alliances, Arctic governance, trade and security partnerships: forced annexation of a territory tied to an allied state would undermine trust and U.S. leadership claims, inviting coordinated resistance; international human rights and legal institutions would amplify reputational damage ^{[3] [2]}.

8. Exceptional counterarguments and their limits

A minority legal argument posits that if the acquiring power controls territory long enough and other states recognize it, legality can follow factual reality — but authoritative sources treat such outcomes as rare and insufficient to justify initial force, and they stress that contemporary law evolved precisely to prevent conquest by force ^{[8] [2]}.

9. Bottom line: practically possible but legally and diplomatically blocked

In sum, while domestic U.S. mechanisms might be mobilized to proclaim annexation, international law treats unilateral acquisition by force as illegal, international institutions and states would likely refuse recognition and mobilize countermeasures, and the political costs to U.S. diplomacy and global standing would be severe — making any credible annexation attempt both unlawful and strategically self‑defeating ^{[1] [4] [3]}.