The predetermined breaking points. The map shows the percentage of the population of Russian origin, according to the 2001 Ukrainian census.
In an ideal world, I would say what I always say in case of conflicts like this, may it be the Basque territory, Catalonia, Ireland, Scotland, Gibraltar, Falkland, wherever: have a referendum of the population in a local region and let them decide, free and uninterferred by external powers, whether they want to side with an external power and become/stay a governed part of that, or be independent. In any case, however, they must pay for their own costs, you cannot be independent while dependign on others to pay your bills. That is a contradiction in itself.
Unfortunately, some counties like the Ukraine lie in the transition zone between "empires", where the influence of the centre fades and the the influence of the other side grows. And empires live by their own logic, that is they cannot continue to exist if allowing being challenged in their periphery without answering to that challenge and winning it. They cannot afford to let such independency movements succeed without loosing themselves in power and relevance.
This contradiciton cannot be solved, and so it all to often gets settled by violence and answering the question who the stronger is. Thats not the ideal life but the real life.
The map shows that 20 years ago the implied Russian argument to have claim for territories due to the Russian share in the regional population only is valid for the Crimean, but neither for Donbass nor Luhansk.
Kiev on the other hand by the Minsk agreement would be obligated to give huge Russian minorities special rights and protection, but has refused to do so, if I can trust the media coverage.