Denmark can't really do anything, as Greenland has been granted self-rule and very wide autonomy, with Denmark retaining control over foreign relations and defence.
Well if you want to be realistic about it, that would be decided by the US military. Greenland is and has been a strategic military posting as far as the US military is concerned going back to WW2. Denmark has a $3 billion military budget, they can't decide or dictate anything when it comes to international politics. NATO would decide whether Denmark were allowed to do anything militarily to retain Greenland, ie the US would decide.
Greenland is loaded with ex-US military bases and the Trump admin has indicated that they want to re-populate one of them. Harder to screw around with land when a different country has a larger military presence on that island than you do.
Because the US has its own islands and if UK/France turn into North Korea then the US would be very quickly to touch said islands.
That said those islands are not in the flight path of Russian bombers and land launched ICBMs.
Greenland and previously Iceland were important strategic assets during the Cold War and with hostilities heating up again with Russia they are becoming more important by the day.
