Russia has invaded Ukraine, but this was a long time in the making. Vladimir Putin is rebuilding the Russian Empire, and this is how he's done it.
Angela Stent, "The Putin Doctrine" https://www.foreignaffairs.com/articles/ukraine/2022-01-27/putin-doctrine
Tom Cotton, "Proxy Wars" https://www.foreignaffairs.com/articles/syria/2015-11-24/proxy-wars