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By the start of World War II , this basic concept had developed into the idea of a " spearhead " , a dense formation of highly mobile forces that would concentrate at a single point , overwhelm them locally , and then run into the lightly defended rear areas .
 World War II
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 World War II