I don’t know what the arguments against this idea would be, but seems to me like the first thing Denmark needs to do, probably with the help of some allies, is expel the US from Greenland, take over the US military base there and strongly fortify it. Then the US can’t use it for the invasion. I’m sure their plans heavily depend on having and using that base. Don’t say anything beforehand, just show up with enough force that the existing US contingent there can’t win if they fight back, present the eviction notice, and get them moving out before US can send reinforcements.