The east of Ukraine has most of their defense production, heavy industrial might, oil, natural gas, and the best farm land. Now it’s pretty much Russia’s again. Putin doesn’t recognize the breakup of the Soviet Union, and wishes he was the head of the Communist Party Soviet Union like Andropov.
Putin came on board after Russia signed away sovereignty of those regions to Ukraine in exchange for relinquishing nukes. We signed that agreement. Russia has violated it twice by invading, the Ukes honored it by giving up their nukes, and we are dealing with a contingent of people who are actively looking to betray the Ukes but keep the nukes.