What legal and NATO mechanisms would be triggered if one member attacked another, and how would they apply to a U.S. attack on Denmark/Greenland?
Executive summary
If one NATO member attacks another, the alliance’s core legal instrument—Article 5 of the North Atlantic Treaty—creates a collective-defence obligation, but it does not automatically prescribe how Allies must respond; activation requires political steps in NATO bodies and may produce non‑military as well as military measures [1] [2] [3]. Applying those rules to a hypothetical U.S. attack on Denmark or Greenland would trigger immediate political and legal crises: Denmark could seek Article 5 consultations and request collective defence, but NATO’s consensus politics and unresolved questions about intra‑alliance aggression mean the Alliance’s practical response would be intensely contested and likely mix diplomacy, sanctions, and non‑kinetic assistance rather than an automatic allied war against the United States [4] [5] [6].
1. What the Treaty actually obliges: Article 5’s legal mechanics
Article 5 declares that an “armed attack” against one member is considered an attack against all and obliges each ally to assist the party attacked by taking “such action as it deems necessary, including the use of armed force” to restore security in the North Atlantic area, but the Treaty leaves the form of that assistance to each state’s judgment and does not mandate an automatic military response [2] [5] [3]. The Treaty and NATO practice also tie Article 5 to Article 51 of the UN Charter—recognising a right to individual and collective self‑defence—and require reporting measures to the UN Security Council [2] [1].
2. How Article 5 is invoked in practice: political process, not automatic trigger
There is no automatic mechanism that lights a fuse when an attack occurs; the North Atlantic Council (NAC) must meet and Allies consult, and the attacked member normally formally invokes Article 5 after consultations—NATO has invoked Article 5 only once, after 9/11, demonstrating its political as much as legal character [7] [1] [5]. Allies may also use Article 4 to convene consultations when a threat is perceived without triggering collective defence obligations, underscoring layered political tools short of full Article 5 mobilization [8].
3. Geographic and functional limits: where and what counts as an “armed attack”
The Treaty contains geographic language that ties obligations to the North Atlantic area, with Article 6 historically interpreted to limit scope, while NATO leadership has modernised practice by recognising that attacks in space, significant cyber operations, or hybrid aggression can amount to an “armed attack” for Article 5 purposes [1] [5]. Thus an attack on Greenland—which is Danish sovereign territory in the North Atlantic area recognized in defence agreements—would satisfy the geographic and material nexus for Article 5 consideration under current NATO interpretations [1] [6].
4. The elephant in the room: what if the aggressor is a NATO member?
The Treaty is not explicit about intra‑alliance aggression between members; historical precedent and legal commentary show uncertainty: Articles of the Treaty do not prescribe a clear course if one Ally attacks another, and scholars and officials debate whether treaty obligations suspend or require other Allies to act against the aggressor—practical politics would dominate [5] [9]. NATO operates by consensus, and a powerful member can block political decisions in NATO bodies; yet defence‑law experts argue that the legal obligation to assist the victim Ally technically persists even if political processes are obstructed, opening space for other forms of assistance short of direct combat against the aggressor [4] [6].
5. Applying the framework to a hypothetical U.S. attack on Denmark/Greenland
If Denmark’s territory in Greenland were attacked by U.S. forces, Denmark could request Article 5 consultations and insist Allies treat the attack as an assault on all; NATO’s machinery—NAC meetings, legal assessments of whether an “armed attack” occurred, and reporting to the UN Security Council—would be triggered immediately [2] [3] [1]. In practice, Allies could lawfully provide varied assistance—sanctions, logistical support, cyber defence, intelligence sharing, or arms—because Article 5 “may, but does not have to, include the use of armed force,” and political constraints would make direct Allied kinetic action against the United States highly fraught [4] [3] [5].
6. Political reality and likely outcomes
Contemporary analysts and sources indicate the likely outcome would be a mix of political condemnation, non‑kinetic measures, and diplomatic efforts to resolve the breach rather than an immediate allied military campaign against the United States; NATO’s consensus rules, U.S. influence, and ambiguity over intra‑alliance aggression mean the Alliance would face an existential political crisis even as legal obligations persist and alternative tools (including EU Article 42 for EU members) could be explored [4] [10] [6].