These kinds of joint exercises are pretty common and largely symbolic.
(The wikipedia page about this contains blatant partisan propaganda. Gross.)
Danish Air force Hercules are doing their best impression of the Berlin air bridge to transport as much as they can to Greenland, airlines have transported personnel, several countries are sending navel ships, planes have already been doing exercises. While this isn't the first exercise there this is absolutely not common and just symbolic. Sure it isn't a force that can fight the US, but it is historic and unprecedented in Danish/Greenlandish history and it is a tripwire.
To put that into perspective the US has around 150-200 staff in Greenland.