Gas wars and refusal to return some military assets they didn’t use themselves but destroyed under the NATO threat reduction program. The persecution of Crimeans was taking place since earlier 1990s as well which was a sensitive topic for the Russian public opinion.
That’s not “belligerant”. That’s Ukraine using what lies within their own national borders for their own self-interest. Ukraine had no obligation to act in Russia’s self-interest rather than their own, and they certainly never threatened Russia militarily.