The origin of the conflict in the Donbass is very simple. Massive reserves of shale gas discovered there in 2011 / 2012, enough volume to make Ukraine something like the 15th biggest producer of gas on the planet.
Russia's two largest export markets are gas and oil and geography has, until now, made Russian gas the easiest for European countries to import.
If Ukraine was to start harvesting and exporting gas on an industrial scale it would negatively impact Russian state finances. So Russia has formented rebellion in the Donbass, making development of the gas field there impossible... And then also seized Crimea, including the territorial waters around Crimea where natural gas and oil was discovered under the sea bed in 2011 / 2012.
Follow the money. It's always about money.