#2 The Spanish grudge
The inaugural UEFA Euro had some notable absentees, including West Germany, Italy and England. However, the Spanish probably had the most politically relevant reason to pull out of the tournament. Spain was required to travel to USSR in order to play a qualification match against the latter. However, the then Spanish dictator was holding a grudge against USSR for supporting the Second Spanish Republic in the Spanish Civil War (1936-39).
Four years later, Spain hosted and won the European Championships. And take a guess who they faced in the final. It was USSR, whom they eventually defeated 2-1.
Back then, this victory was translated as the triumph of Fascism over Communism. But all we saw were the curious ways of the footballing deity.