Ukraine was under Russia's governance, Maidan unlinked the governance, so Russia's governance fed Dombas separatism and sent undercover troops (aka destabilization and progressive control), Ukraine's new governance responded with troops. Time passed, Russia's governance got inpatient, sent regular troops thinking it would be a way to finish and regain quickly the control over Ukraine's governance, aka open invasion, but the troops didn't know they were being sent into a war, and the external support was underestimated.