Sure... there is always a chance for two democracies to go to war but it would most likely be a rather limited war at best. Most likely it will not happen and issues will be dealt with by diplomacy and pressures from other democratic countries for the parties to solve their issues peacefully.
As new generations are born and social injustice is reduced so will cultural issues vanish as well with it. As long as people feel secure and happy they will eventually set aside old grudges and start cooperate instead. This will be true everywhere... the key to having a stable Democracy is not forcing it down he throat but to encourage their citizen to change into it by increasing their education and economy at the same time thus increase the living standards.
China is not yet a democracy... but in my opinion China is closer to becoming one than Russia at the moment... if China is going down the same road it has the last decades it will most likely turn in to a relatively stable democracy sooner rather than later... that is my guess. I actually don't think China will be such a threat to world stability in the future even though they are pretty aggressive in their policies for the time being. But that is my personal opinion.
Mostly agree except for one thing. Cultural issue will always exist. This is because humans can only really care about around 10 to twenty people, and acknowledge a few more than that. Past that we start to see them as competitor and foes in some way shape or form, or as someone to be exploited in some fashion.
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