If you were to trace what gives a US President the idea that he...

If you were to trace what gives a US President the idea that he can forcibly annex Greenland, I think it's more likely the weakness of Europe's response to Russia's annexation of Crimea in 2014. That's the point at which a "Great Power" that places national interest above all else tested international resolve. Eight years later when the Russians went after the rest of Ukraine Germany was still dependent on Russian gas to stop its population from freezing in Winter, rendering full scale sanctions impractical. That leaves someone like Trump ruminating on what the implications are if he moves on Greenland and believing that there aren't any. Would the EU ban all trade with the US, freeze all US assets in Europe, prohibit US nationals from travelling to Europe other than to seek asylum, and direct their nuclear warheads at the USA to ensure Mutually Assured Destruction in the event of a hostile American response? You never want to be the "reasonable" partner in a relationship with someone who's crazy.