24123
Throughout the late 1950 s and into the 1960 s , the United States and the Soviet Union had been developing missile systems with the ability to shoot down incoming ICBM warheads .
 United States, Soviet Union
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 United States, Soviet Union