It’s a long complicated answer. But this link helpsWhat would be the objective for Russia to win over Ukraine or vice versa?
Why does Russia want Ukraine?
A University of Rochester expert on international conflicts explains some of the reasons behind Vladimir Putin invading Ukraine.
www.rochester.edu
Putin was not happy with the fall of the Soviet Union and wants to re-establish that system or replicate it on some other fashion so long as it is an autocratic system and not democratic. From a timing perspective, Russia invaded Crimea and Eastern Ukraine back in 2014 and has been at war, without ever declaring war, ever since. The West stood back for the most part. With Trump working to weaken NATO and threatening to get out altogether Putin felt more emboldened that he could invade and NATO and the West would sit back again. Second, Putin’s own existence and control of power is based on Autocratic rule. With more and more Democratic systems appearing right on his doorstep the offers visible alternatives to the Russian people that might question autocratic rule and Putin maintaining power. The fact that many of those are NATO countries right on his borders that puts more pressure on him and raises tensions.
Ukraine wants democratic rule and to join NATO. They were invaded and are fighting on their own land to repel the invaders and recapture the lands lost in 2014. This gives the Ukrainians a big advantage. When you fight for you own lands you will fight harder and never give up because where else will you go.
It will be a long war and one of attrition. So long as Ukraine can keep getting ammunition and weapons from the West, it is only a matter of time before Russia loses. The more troops and equipment Russia loses and cannot replace, the weaker they become elsewhere.