It all depends on how far back you want to go. Before the annexation, there was a referendum, but before the referendum, there was de facto occupation by the "polite people" aka Russian forces with all identification tags removed (a war crime in and of itself, by the way).

Before the occupation, there was Maidan. And before Maidan, there was Yanukovich, whose achievements include stuffing the Supreme Court, and having it scrap the country's new 2004 Constitution and reverting to the old one, which, coincidentally, gave president a great deal more power than the new one.

We can go all the way back to Pereyaslav Rada if you want, but what's the point of the exercise? No matter how you slice it, Russia invaded and occupied a part of a sovereign country, and there was no genocide or other serious crimes against humanity ongoing there that could possibly justify such a thing.