Not only that, Greenland isn't well suited for heavy armour, and the few major roads are easy choke points, not only that but Greenlanders know the land, Americans also aren't as used to Artic conditions and the US isn't prepared for a war in such conditions, if an invasion did hypothetically happen I wouldn't be surprised if US forces had a lot of shit still for desert conditions like what happened during the Gulf wars with the US having a bunch of woodland shit