2014 ... Obama / US / West / CIA (aka Jєωs) overthrew the pro-Russian government and installed Western puppets.
That puppet government immediately started attacking the Russian populations in the East of Ukraine. Jєωs as per usual drew up borders that split various populations off from their true ethnic ties (they've done this over and over again), so you had Russians who were basically stranded in Eastern Ukraine.
Crimea (also mostly Russian) voted to separate from Ukraine and rejoin Russia, and Putin recognized this.
Putin tried to aid those populations in East Ukraine and worked the Minsk agreements in early 2015, but the Ukrainian government never kept it and have been attacking the Eastern Russians ever since.
In the meantime, NATO keeps adding new members moving East and encroaching upon Russia including offering to let Ukraine join, thus having NATO right up against Russia's borders.
Between the continuing attacks on the Russian populations in East Ukraine and the refusal of NATO to back away from Russia's borders, and constant hostile rhetoric from the West (for no real reason), Putin finally declared the Eastern Ukrainian populations to be independent republics and quickly signed mutual aid agreements with their representatives.
So Putin sent in "peacekeeping troops" at the request of those populations, and so from his perspective there was no invasion of Ukraine but coming to the aid of an ally at their request.
Probably smelling retaliation coming through Ukraine, Putin then moved to take out the Ukrainian military (including possibly bioweapons labs funded by the US and a secrete nuclear arms stash hidden at Chernobyl). Reports are that he effectively neutralized them in about 90 minutes.
US propaganda has him "invading" Ukraine (rather than making limited defensive operations) and bombing civilians. Putin is not attacking civilian targets although there undoubtedly have been civilians killed by now with all the military activity.