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What would be the earliest possible date for a state that has 99-100% of global surface area under its both De Jure and De Facto rule. So that as far as people are concerned its not just a random ruler but someone that is in general accepted as being rightful (so UN is both too unpopular and too weak to count) but a state like the EU would be accepted.
So. What is the earliest possible date at which you can fathom a entirely unified earth. I cant think of any pre1900 POD's that result in whole world. PoD's can be as far back as you can think of but unless you have eqyptians living from both poles to the equator, you dont have unified ancient eqypt even if its the only civilization.
My best bet for a unified earth is some kind of communist revolution during the 20th century. Only time it could conceivably happen. Other possible times are US that goes batshit insane after 911 and A cold war that turns into a all out match with very little nuclear weapons used for some reason ending in global conquest.