Because it's on Russia's doorstep, was part of Russia for 400 years until 1991, has a large Russian minority, the Russian Black Sea fleet is based in Sevastopol, the Russia-friendly government had just been toppled in a coup backed by the US and Western European powers, and Russia didn't want another NATO member on its doorstep.

I can't see what would possibly lead to a Russian invasion of Denmark.

> in a coup backed by the US and Western European powers

Seriously? I'm sure the fact that Ukrainians could see contrast with next-door Poland had nothing to do with Maidan... no, it was CIA, definitely.

They can also see the contrast with next-door Russia. The Ukraine is one of the poorest countries in Europe.

The overthrow of the government was effected by armed militias, with a heavy far-right presence (e.g., pictures of Stepan Bandera, the Nazi collaborator who ethnically cleansed Poles from Western Ukraine during WWII). The US government had a hand in picking the successor government - there's even audio of the behind-the-scenes discussions. There was obviously popular support in Western Ukraine for Maidan, but there was also a lot of foreign involvement and the seizure of power itself was violent and unconstitutional.

For tons of historical and current reasons.

From Crimea being part of Russia for centuries, having 60% russian ethnicity population, Ukraine government being toppled by nationalists and neo-nazis with US support, etc...

Because Putin historically has boosted his popular vote by creating a foreign enemy and being the tough guy.

And in that context, is it really impossible that Russia would decide e.g. to annex Bornholm, if they would deem it feasible?

https://www.thelocal.dk/20150625/russia-rehearsed-takeover-o...

Remember, Ukrainians were literally "brothers"; if it happened to them, is any weak neighbor really safe?