The Battle of Los Angeles
In an incident now known as "The Battle of Los Angeles", the U.S. Army fired several thousand anti-aircraft shells at an unidentified target over Santa Monica, California during the night of February 24-25, 1942. The target was later officially determined to be a lost weather balloon, although this was never confirmed.
The San Francisco Bay Area on alert
In May and June 1942, the San Francisco Bay Area underwent a series of alerts:
May 12: A twenty-five minute air-raid alert.
May 27: West Coast defences put on alert after Army codebreakers learned that the Japanese intended a series of hit-and-run attacks in reprisal for the Doolittle Raid.
May 31: The battleships USS Colorado and USS Maryland set sail from the Golden Gate to form a line of defense against any Japanese attack mounted on San Francisco.
June 2: A nine-minute air-raid alert, including at 9:22pm a radio silence order applied to all radio stations from Mexico to Canada.
There was also a forty-five minute air-raid alert and radio silence order later in the year, on November 28.