So I just realised that the EU has a common defense clause, Article 42(7).
If the Hague invasion act was actually invoked by Trump, or if he just straight up tried to invade a territory of a member state, the entire EU “shall have towards it an obligation of aid and assistance by all the means in their power”, a clause that requires immediate compliance (no centralised approval necessary) and that seemingly in opinion of the European Parliament, superseeds NATO itself, not to mention that it asks far more of its members (all the means in their power).
If Trump actually decided to invade Greenland or something else like that, Denmark could totally invoke Article 42(7) and have the entire European Union obligated to send what they can.
This is like when Nazi Germany attacked a few colonial powers and then triggered a world war because all of those colonies declared war as well.
Obviously, the EU doesn’t really have the force it needs to stand up to the US, but I’m sure China wouldn’t miss out on the opportunity to redefine the world order in its favor.