Here are three. It is complicated and you might want to research it some in qresear.ch - ignore the message saying it isn't working , it is.
Ukraine used to be part of the old USSR. Some areas speak Russian and wanted to be part of Russia. Other part of Ukraine hated Russia and are very anti-communist & supported Hitler's Nazis during the war.
Anyway, maybe these will help:
Ukraine approves strategy to “recover” Crimea, threatening all-out war with Russia
https://www.wsws.org/en/articles/2021/03/20/ukra-m20.html
Ukraine's President heads to the trenches as Russia masses its troops
https://www.cnn.com/2021/04/12/europe/ukraine-zelensky-front-lines/index.html
Veteran Intelligence Professionals for Sanity on Avoiding War in Ukraine
http://www.ronpaulinstitute.org/archives/featured-articles/2021/april/07/veteran-intelligence-professionals-for-sanity-on-avoiding-war-in-ukraine/