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Before the attack on Pearl Harbor, the 800,000-member America First Committee vehemently opposed any American intervention in the European conflict, even as America sold military aid to Britain and the Soviet Union through the Lend-Lease program. Opposition to war in the U.S. vanished after the attack. On 8 December, the United States, the United Kingdom, Canada, and the Netherlands declared war on Japan, followed by China and Australia the next day. Four days after Pearl Harbor, Nazi Germany and Fascist Italy declared war on the United States, drawing the country into a two-theater war. This is widely agreed to be a grand strategic blunder, as it abrogated the benefit Germany gained by Japan's distraction of the U.S. (predicted months before in a memo by Commander Arthur McCollum)[nb 12] and the reduction in aid to Britain, which both Congress and Hitler had managed to avoid during over a year of mutual provocation, which would otherwise have resulted.

What group was opposed to American involvement in the war in Europe?
America First Committee