Buddhism becomes the dominant religion in Asia in the years before the birth of Christ. Meanwhile in the west, the Roman Empire survives while Jesus dies insignificantly without ever being crucified, and his followers are a very small minority in the Empire that continues to shrink throughout the centuries.
Buddhism becomes a significant religion within the Empire competing with Polytheism. Meanwhile in the east, China becomes a superpower and India breaks into warring city-states that are mixes of Hindu and Buddhist. Muhammad is born into an Arabia that is a mix of Jewish and Buddhist and later founds a slightly different version of OTL Islam. Islam becomes the dominant religion of parts of Africa and the Middle East, and its armies take control of the old Persian empire, with the east almost totally Buddhist, Europe and North Africa a mix of Polytheism, Paganism and Buddhism, and the Americas still practicing tribal religions.
Three major superpowers emerged, the Roman Empire, the Arabian Caliphate and China. Rome fights a constant series of wars with the Arabs on eastern borders of the Empire. Meanwhile in Europe distinct barbarian kingdoms begin to emerge beyond the borders of the Empire, some allies and some enemies of Rome. China expands into Southeast Asia, Mongolia, Korea, and Japan. By 900 A.D. northern India is united under one empire while the south still is divided. In the Americas the Inca, Aztec and Mayan empires begin to emerge.
In the year 1232 A.D. an Arabian ship headed for Africa gets blown off course in the Atlantic and ends up in the Caribbean. When news of this gets back to Arabia and the rest of the eastern hemisphere Rome, Arabia and barbarian kingdoms such as Alemannia, Saxonia, Anglica, Gotland, Hibernia, and Francia all launch colonial expeditions to the Americas, with varying degrees of success.
(To be expanded and continued)