And is as legitimate as it gets.
Yanukovych's de facto removal from office was a coup.
His own party has voted against him after he tried to sell Ukraine to Mr Putin.
A coup is something of much smaller scale, not result of a massive nation wide protest.
And some part of the population in Crimea wants them there.
Some population in Kaliningrad wants Germans there.
Some population in Kuril islands wants Japanese there.
Some population in Siberia wants Chinese there.
And oh, some population in USA wants Russian there too. Shall they?
So, an "invasion"? Not clear.
That's what illegally sending troops into foreign country is.
As for "an existential fight in the west", it's doubtful that Putin wants to absorb all of Ukraine.
There is no doubt he wants as much of it, as he can grasp. He claimed USSR collapse to be a "big tragedy" and from time to time whines about territories lost by collapse of russian Empire back in 1918.
Keep in mind that Ukraine is a synthetic state, based on the "Ukrainian Soviet Socialist Republic" set up by the USSR...which was created with a bunch of ethnic Russians exactly to keep Ukrainian nationalism in check.
The biggest country in Europe with 44 million citizens (about 80% of them Ukrainians and "whopping" 17% Russians) is "synthetic" eh?
Are you Russian, my friend? Because I've only heard BS like that coming from pro-Putin Russians.