After the Japanese attack on Pearl Harbor in December 1941, and the entry of the United States into the Second World War, many in California feared a Japanese invasion. In the wake of the attack, Olson urged calm from Californians. In a plea for racial tolerance, broadcast on December 14, he stated he had assurances from "every racial group" of their loyalty and devotion to the United States, even reading a telegram he had received from a Japanese citizen. Olson attempted to revoke the business licenses of "enemy alien" Japanese in California. (Japanese immigrants were prohibited by law from becoming U.S. citizens and were therefore permanent aliens, although many had resided in California for decades.) On February 19, 1942, President Franklin Roosevelt issued Executive Order 9066, allowing U.S. military commanders to create zones from which "any or all persons may be excluded." Based on that, all West Coast Japanese Americans, including American-born Nisei and Sansei, in addition to the non-citizen Issei, were forcibly relocated to isolated internment camps over the next several months.