The American audience reacted to Godzilla differently because they did not recognize the monster was an allegory for atomic bombs.
This movie is about the appearance of a monster that threats to destroy everything. This movie was released in 1954 in Japan and an American movie was released two years later.
In Japan, the movie appealed to the atomic bombs detonated in Hiroshima and Nagasaki in 1945. This means Godzilla represented the bombs and the possibility of destruction due to them.
On the other hand, in America Godzilla was just a fictional monster and it did not appeal to a particular event.
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