Defending #Greenland in case of military aggression from the USA is often reduced to the complexities of Art. 5 NATO. But there is a similar rule for EU member states, that will be automatically triggered without NATO but demands the same commitments as given to NATO for EU members that are also NATO members.
Article 42(7) of the Treaty on European Union (TEU)
https://www.eeas.europa.eu/eeas/article-427-teu-eus-mutual-assistance-clause_en