My understanding is that the genocide started in 2014 right after the US coup on Ukraine. The Nazis in power did not like the Russian people who were living in the east and south of Ukraine so they started killing them. There are reports of 14,000 being killed over the eight year period since then.

Ukraine agreed to stop doing that and signed the Minsk Agreements but then they did not implement them and the killing continued. The Russians in Crimea and Donbas wanted out and, I believe, they held referenda confirming that. Russia intervened in Crimea straightaway but Donbas (Luhansk and Donetsk) was left behind so the shelling and bombing continued.

Again, rumour, but I heard that Ukraine was going to invade Donbas in March so Putin pre-empted them to try and stop the genocide that the rest of the world was ignoring.