The Soviet Union was the country moving troops to assist the Allies in invading Japan's largest islands before the atomic bombs were used.
The country that was moving troops over to assist the other Allies in an invasion of Japan's largest islands when the atomic bombs were used was the Soviet Union. As the war in the Pacific reached its climax, the Soviet Union declared war on Japan and launched an invasion of Manchuria, which played a significant role in Japan's decision to surrender. This move by the Soviet Union added pressure on Japan and hastened the end of World War II.
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