President Franklin D. Roosevelt called Dec. 7, 1941, “a date which will live in infamy.” And that day, when the Japanese launched a surprise attack on the U.S. naval base at Pearl Harbor, has lived in infamy for 70 years. Yet even as the memory of the attack has lasted, so have the misperceptions surrounding it. On this anniversary, here are a few myths worth dispelling.

1. The U.S. government had no knowledge of a potential Japanese attack before Dec. 7.

Beyond the obvious signs of Japan’s increasing aggression — including its sinking of an American naval vessel in the Yangtze Riverand its signing of the Tripartite Pact with fascist Italy and Nazi Germany — various specific war warnings had been sent by...