The Danish government has announced a huge boost in defence spending for Greenland, hours after US President-elect Donald Trump repeated his desire to purchase the Arctic territory. Danish Defence Minister Troels Lund Poulsen said the package was a “double digit billion amount” in krone, or at least $1.5bn (£1.2bn). He described the timing of the announcement as an “irony of fate”. On Monday Trump said ownership and control of the huge island was an “absolute necessity” for the US. Greenland, an autonomous Danish territory, is home to a large US space facility and is strategically important for the US, lying on its shortest route to Europe. It has major mineral and oil reserves.
It’s less that and more the rest of article 5 which lets each signatory decide what level of response is appropriate.
So really, the question you should be asking, is if the US invades, which other signatories would take armed action and declare war against the US, which I suspect is probably not ‘everyone’.
Greenland is Danish territory which means that the EU defence clause applies which has no “let’s see what we want to do” carve-outs, it’s “obligation of aid and assistance by all the means in their power,”. All means. Which means that suddenly French nukes are on the table.
If the US want to have Greenland there’s exactly one reasonable way to have it: Convince Greenland to declare independence, which under Danish law they have the right to anytime they want, secondly, convince Greenland to join the US.
Adding to the French nukes part, while France is generally considered a weaker nuclear power, their nuclear doctrine gives them a warning shot to demonstrate their deterrence capability and to notify an opponent that they crossed a line. If the USA were to invade Greenland, french nukes could theoretically be used without retaliation.