There has never been peaceful coexistence among humans in all of recorded history, so there is little reason to believe there ever was. The reasons for this are many, but we can speculate that in the beginning, violence between people was driven by hunger, a need for shelter and sometimes simply greed. In later times, when people came together in larger groups and began cooperating so that everyone in the group had shelter and enough to eat, the only way to perpetuate and protect that society was to create a set of rules (or religion) that applied to all. That way, everyone knew what was acceptable and what was not. Enforcing the rules and ensuring the religion was followed, lead to the evolution of a hierarchy (or government) in each society. By various means, including fear tactics and especially by limiting education to a select few, these enforcers could ensure that everyone conformed.
Unfortunately, if one society encountered another that had a different set of rules (or religion), their absolute belief and conviction that theirs was the "right" one, could, and often would, lead to war.
Of course this description is very basic and simple. There are many complex dynamics that contribute to the strife we still see among differing sects and societies; the point or question that I really want you to consider in this post, is how can we reconcile our societal differences in a peaceful way, when all of the participants believe passionately that theirs is the right way and everyone else is wrong?
The answer is education, not subjugation. If we can educate the populations of those societies that are in need of more food, or better living conditions, or that have oppresive governments, we will have given them the tools to build strong, healthy, more peaceful societies. The problem lies with the enforcers - they don't want to lose their positions of power, so they will resist any attempt to educate the general population.
Because we live in the age of instant communication and we have the ability to know immediately what is happening anywhere on the earth, for the first time in our history we are a position to reach all people. It is not our religious beliefs we need to be sharing; instead we need to be educating those subjugated or poorer societies. We need to be building schools and spreading our knowledge of medicine, agriculture, industry, history and the arts. This will take a few generations to come to fruition, but eventually, through education and interaction with the rest of the world, we can adopt and adapt to each others' best traits and begin to think as one society instead of the sadly fragmented and secular groups we have been historically and still are today.