List of Olympic Games boycotts
While all countries are normally eligible to compete in the Olympic Games, there have been several instances of nations boycotting or being barred from participation, often in protest to decisions made by the International Olympic Committee. Austria, Bulgaria, Germany, Hungary, and the Ottoman Empire were not invited to attend the 1920 Antwerp Games in Belgium, having lost World War I. South Africa was not invited to the 1964 Games, while its invitation to the 1968 Games was withdrawn after several other African countries threatened to boycott the Games due to apartheid.
Source: Wikipedia — List of Olympic Games boycotts (CC BY-SA 4.0)