Definitely not legal but if Congress and SCOTUS won't stop him, then it is what it is.
Remember: The Constitution is just a bunch of promises we agree to keep. If the promises start to get broken and no one pushes back, it's not worth the paper it's written on.
Trump threatening to take Greenland.. (and Canada too in less aggressive terms) and perhaps by force as he indicates sending more troops there.. very interesting so far, didn’t think we’d see that kind of rhetoric.. is that even legal lol