The conflict started more than a decade ago. Currently it's a proxy war mostly limited to the territory of Ukraine and Russia. Other countries are in the pipeline.
The next target after Russia loses against NATO will be China actually. And then Iran, North Korea. But you'll get a total nuclear exchange well before, so it's academical.
So likely next proxy is Moldova, Romania, Georgia, Belorus. Poland is also a possibility, but not at first. Unless Belorus, but then we're at tactical nukes stage already.
Russia was the aggressor here. Ukraines wanted to go towards the west for a better life and protection from being further invaded by Russia. Putin is of the school of thought that Ukraine isn't real. But it is the people of Ukraine who decide that.
The overthrow of a democratically elected president? That is what caused Eastern provinces to go into rebellion. Forcing those Eastern provinces to go back to being ruled by a pronato kyiv is impossible and will just lead to more fighting that will resolve nothing