He wants to control Ukraine's government. What they did for years after the breakup of the USSR was to sell gas at half price to one person in the Ukrainian government, then he resales it at regular price and uses the slush fund to pay bribes to do what Russia wants. One lady was able to get rid of it for a while, but she was removed from office and put in prison.
When the USSR broke up, we did promise them we would not expand NATO around the old Soviet States, and we did. But when Ukraine gave the nukes they had on their soil back to Russia, Russia signed a treaty to recognize and respect their borders. Which they did not do.