It is 100% Putin's war. He didn't even inform the Russian FSB before moving in. All the blablabla about Nato, the US "choosing" Ukraine's leaders is literally repeating Kremlin talking points and doing a great disservice to the Ukrainian people who have a better grip about what is going on in their part of the world than most foreign experts. Fact of the matter is, Ukraine wanted control over its own destiny and not to become a Russian vassal state like Belarus, constantly having to clear any foreign policy points by Moscow before proceeding.
Remember, Putin's meddling in Eastern Ukraine started many years before Nato etc was even a topic of discussion. Ukraine was not keen on joining Nato nor becoming beholden to the US in any way. Europe was the direction it wanted to move, in order to become a "normal" country. Elections in Eastern Ukraine were constantly subject to dubious counts, pro-Ukrainian candidates beaten up by groups of "silovniki" or otherwise excluded from the democratic process. Moscow's installed puppet Yanukovich constantly undermining Ukraine's independence - reversing any policy of supporting the Ukrainian language and education and prioritising whatever policy emanated from the Kremlin, whilst libelling all Ukrainian speakers as nazis. This started years ago before most people were even aware Ukraine existed.
It all kicked off when Yanukovich went back on an election pledge to forge closer ties with the EU and instead did a sudden volte-face deciding to join Putin's vainglorious and economically damaging customs union. There was zero support for that anywhere and it led to tents and protests. None of this was out of the ordinary for Ukraine, which had a reasonably robust democratic setup with free reign to protest by eastern European standards. It all went badly wrong, when Yanukovich, on Moscow's orders, sent the boys in to beat the students up.
So here we are.