Final answer:
President Roosevelt was referring to the surprise attack on Pearl Harbor.
Step-by-step explanation:
When President Roosevelt stated that this event was "a date which will live in infamy," he was referring to the surprise attack on Pearl Harbor. On December 7, 1941, the Japanese conducted a surprise military strike on the United States naval base at Pearl Harbor in Hawaii. This attack propelled the United States into World War II, causing a significant impact on American history.
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