This is part of a world descript for a science fiction setting. If you got here by accident, hit the back button on your browser.
WW3 broke out for a variety of reasons: terrorism and related instability, income inequality and inaccessibility to necessary resources (such as healthcare and clean water), and an ongoing refugee crisis. Essentially, the world faced open rebellion and continuing conflict. Countries defaulted to old loyalties, and the world devolved into open warfare to the point that almost nowhere went untouched. I’m not going to go into too many specifics here on the war itself since I’d father focus on sorting out the setting details pertaining to the history immediately after.
When almost everyone was finally weary of fighting, what leaders were left in the world met in Geneva (in honor of the Geneva convention, since it was believed that the historical humanitarian connection would bolster their credibility). They hammered out an agreement for one unified nation of the world under what they referred to as the more humane system. Instead of separate countries (which, by this point, were seen mainly as hotbeds for harmful nationalism, misallocation of resources, and divisive alliances), the world would be united into a single nation. The former individual countries would become like states with a newly formed worldwide government functioning like the US’s federal government (even closer than countries in the EU). Certain localities such as Catalonia, Kurdistan, the American South, and Scotland were granted the same state-like status. Resources were to be allocated based on need, and all currency was abolished. At this point, major portions of the world have devolved to a barter system or community self-sufficiency, so it was actually easier to abolish currency rather than try to convert what was left into one unified system. For details on the economy and politics that resulted, see that link (off the main Balance of Souls world descript).
Although certain allegiances fought this more humane system (yes, terrorism still existed), most people saw it as an ideal to reach for. The more humane system was set out in a series of points distributed freely in all major languages across the internet. They saw unprecedented individual and community action in favor of this new model of government. Against all odds to the contrary (such as unstable regions, allegiances, propaganda, and the like), the more humane system was victorious, and the world entered a stage of peace. Historians by time point two speculated that the world was ready for an end to the conflict. The resulting government leans what our modern day would call leftist, but they still lack one unified ideology.
Although the majority of the above probably seems a bit vague, that is because I don’t want to accidentally limit later stories by creating specific players or major battles. This is just for reference for their basic history.