Certainly, here are 3 globally impactful dates in history that happened before the year 1000:

1. 10 or 11 June 323 BC: The death of Alexander the Great, king of Macedonia, and later also leader of Persia, Egypt, the Hellenic League (modern day Greece) and Northwestern India, which he conquered, creating one of the largest empires in history. His death marked the end of this empire, and the start of the Hellenistic period. The Hellenistic period saw great worldwide influence of Greek culture, and great progresses in art, literature, philosophy, architecture, music, mathematics and science.
2. 6 July 247 BC: The start of the reign of Qin Shi Huang, who would go on to unify all of China and become the first Chinese Emperor.
3. 29 September 440: The coronation of Pope Leo I "The Great", one of the most influential heads of the Catholic church, famous for convincing Atilla the Hun to end his invasion of Italy in 452.
4. 4 September 476: Odoacer's deposition of Romulus Augustus, the last West Roman Emperor. By convention, this is the date that marks and finalises the 2 century long decline and eventual fall of the Western Roman Empire, which had been the dominating political power in Europe, Africa and Asia Minor for over four centuries.
5. 25 December 800E: Pope Leo III crowns Charlemagne as the emperor of the Holy Roman Empire, creating one of the greatest political entities in Europe until its abolition in 1806.

Note that as you go further back into history, dates become increasingly less certain. For many important world events, we cannot determine their exact date, and thus they are absent from this list. For this reason, when looking at important single dates, lists will skew towards more recent events.
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