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The invasion started in 2014 and the reason was Ukraine's intention to join the EU. That was an existential threat to Putin. Ukraine's free and fair multi-party democratic elections were already a threat to him. The people's choice of EU membership was the last straw. If Ukraine had prospered the way Poland did when they joined the EU, Russians would have wanted rid of Putin and dictatorship.
The reason for the Russian invasion of Ukraine is NATO inaction: inaction after the invasion of Georgia, inaction after warcrimes in Syria, inaction after the annexation of Crimea. NATO has stood by and done nothing, effectively inviting Putin to continue.