Some time back, Mike Pompeo, the Secretary of State, was blathering on about the benefits of global warming - yep, you heard me right. One such benefit, according to the Secretary of State, is that as the ice around the Arctic melts, so the North-West Passage will open up, allowing sea-going traffic from the North Atlantic to access the Pacific Ocean without needing to pass through the Panama Canal.
Problem is, the Russians, who are just over the other side of the North Pole, have been making noises about having sovereignty over much of the Arctic region. The U.S. would similarly have a sovereignty claim to large swathes of the Arctic region if they had more territory in the region in (they already have Alaska).
Now, remember that Trump suggested the Russians had a sovereignty claim to Crimea because many of the inhabitants spoke Russian and wanted to be part of Russia. Well, expect similar reasoning, though leaning towards Denmark (Greenland's protector) being far away from Greenland and unable to protect Greenlanders from those pesky Ruskies. The US will justifying annexing Greenland by claiming to be the more logical protector, and emphasise the importance of Greenland being a US possession for the security of the Free World, blah, blah, blah.
Anyhow, let's see how the annexation of Greenland plays out!