They were cozying up to the EU for a long time. Putin warned them over and over. This isn’t the only reason, but Russia have them many chances to back down. They chose to keep poking the bear.
And, they stiffed him on a couple billion while getting the money on the other end from Germany.
Note to Ukraine: When taking a crap on someone’s front porch, and ringing the doorbell, make sure you leave the car running.
So what. Every nation deserves to be able to make its own choices.
Ukraine's mistake was giving up its nuclear weapons.