It's still mind-boggling that the official position from Russia at the time was that they were not involved, and troops were just on vacation. And the wester governments were like hur-dur-dur, I'd like to speak to the manager.
From what I remember it started out as a civil conflict. Kyiv neglected Crimea and the local government and police became hostile. The separatists claimed some key infrastructure like the water ways and such but not too much happened. Then they got unbadged Russian troops to help. And soon afterward Russia just took the movement over and annexed the whole thing.
I definitely remember hearing numerous stories leading up to the event of a bunch of Russian speaking agitators that were not locals and suspiciously well supplied weeks before the invasion began and troops were spotted 'on vacation'.