

Armchair general? NATO’s Article 5 is a collective defense pact: an attack on one member is an attack against all members. With the US being the aggressor, Denmark and Greenland would be the members invoking Article 5, and surely all other NATO members would enter the war against the US.
Trump could invoke Article 5 too I assume, but I doubt (I fucking pray) any NATO member would ally with the US.
I recommend you read this.