NATO promised it would not put troops or weapons into balkan territories after the breakup of the Soviet Union, but Russia did not get it in writing. They now have troops and weapons in all those place, and have been trying to court Ukraine to join NATO. To make this easier, CIA and MI6 basically staged a coup in 2014 in Ukraine, by inserting a NATO friendly candidate who 'won'. Putin took Crimea during the ensuing shenanigans, because it is nearly 100% Russian people and holds their southern Naval base. Now it is the eastern regions of the country he wants to secure, to prevent NATO from being able to advance right up to his fucking doorstep.
Military logistics on this is that he doesn't want our future hypersonic missiles to be just minutes away from Moscow, which they would be. Which is why Russia wanted those countries as a buffer in the first place.
Beyond that, Ukraine is a cess pool of corruption, no doubt filled with financial records, bio labs, and perhaps data centers that the US and NATO would love to destroy. They are making every possible effort to fuck things up for Putin, destroy evidence, and sneak Ukraine into NATO membership, without actually committing ground forces.