alt Hacker NewsOne similarity is, if I'm correct, Russia claimed that the naval base of Sevastopol was vital for Russian security.
The protection of the population and the illegitimacy of the current government was an argument developed by Russia at the time, it has not been yet by USA but I suspect this might start develop in the next few weeks.
The common ingredients to justify an invasion/annexation is a mix of: - Self-Defense, security - Historical, Geographical claims - Protection of the population - Moral sugar coating (we had no choice)
Greenland was not part of the USA for a 150 years. It is also not mostly populated by ethnic Americans who speak English as their mother tongue. Nor is there a large American military base there that Greenland / Denmark is trying to evict them from and potentially hand over to a geopolitical adversary. Greenland is also an island a couple thousand miles away from Denmark.
Honestly, it is such a surprise that the difference is not obvious.