What is NATO’s Article 4, could Denmark invoke it to protect Greenland from the US?

The question of Greenland’s security has become a matter of international concern. US President Donald Trump has been repeatedly stating that Greenland is vital to American national security and has refused to rule out military action to assert control. According to a report by Daily Mail, Trump has asked senior military commanders to prepare contingency plans for possible invasion of Greenland. His remarks have alarmed both Danish and Greenlandic authorities and raised questions about alliance norms within NATO. In response, Denmark has warned that any aggressive attempt to seize Greenland would meet immediate resistance.

If US President Trump were to press ahead, the question of whether Denmark could invoke NATO’s Article 4 would come into sharp focus. The article states: “The Parties will consult together whenever, in the opinion of any of them, the territorial integrity, political independence or security of any of the Parties is threatened.”. Such consultations are intended to evaluate the nature of the risk and coordinate a collective response among allies. Crucially, Article 4 does not mandate military action; it serves as a diplomatic and strategic mechanism for discussion within the alliance.

According to NATO,"Under Article 4 of NATO’s founding treaty, members can bring any issue of concern, especially related to the security of a member country, to the table for discussion within the North Atlantic Council." Since the Alliance’s creation in 1949, Article 4 has been invoked nine times. Any member country of the NATO can formally invoke this article. As soon as it is invoked, the issue is discussed and can potentially lead to some form of joint decision or action on behalf of the Alliance.

On February 10, 2003, Turkey formally invoked Article 4, asking for consultations in the NAC on defensive assistance from NATO in the event of a threat to its population or territory resulting from armed conflict in neighbouring Iraq. NATO then agreed a package of defensive measures and conducted an operation named, Operation Display Deterrence from the end of February to early May 2003. Additionally, Turkey has invoked this article about 4 more times in 2012, 2015 and 2020.

On September 23, 2025, the North Atlantic Council met following Estonia’s request for Article 4 consultations after three Russian fighter jets violated Estonian airspace on 19 September 2025.

In September 2025, Denmark said that it was considering whether to invoke Article 4 after drone incursions temporarily forced the closure of two airports and disrupted military installations in the western Jutland region overnight, Reuters reported. Earlier, Prime Minister Mette Frederiksen had linked similar drone incidents to a wider pattern of suspected Russian drone activity across Europe, allegations that Moscow has denied. Despite these developments, Denmark’s foreign minister later clarified that the government currently has no plans to invoke the Article.

In contrast, Article 5 obliges NATO members to consider an attack on one member as an attack on all. It is one of NATO's core principles. In response, each other member would take "such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area". It has been invoked only once, following the devastating 9/11 attacks on the World Trade Centre and the Pentagon in 2001. While powerful, Article 5 relies on consensus among member states before collective action is taken.

Despite the US threats, Denmark has so far indicated that it has no plans to invoke Article 4. Observers note that Trump’s statements echo Cold War-era behaviour, when the Soviet Union intervened in allied nations under the Warsaw Pact to enforce control. Analysts warn that any US move against Greenland would risk undermining NATO cohesion and could provoke a consultation under Article 4.

If Denmark invoked Article 4 citing a threat from the US, the alliance would be obliged to discuss collective responses. Only if hostilities escalated might Article 5 be considered, obliging members to support Denmark militarily.