The disrespectful, colonialist treatment of Greenland by Denmark is sufficiently well documented. And if you think Europe stands a chance in a war against US then I don't even know what to say anymore ...
So yes, defending Greenland becomes a case of helping a people stay free and not invaded, no matter the enemy.
It's silly to say "well we have no chance against" because then you can end that with China, Russia or even India.