Because of the distance from the European, African and Pacific campaigns, mainland United States never became a significant site of battle during World War II.
But on February 23, 1942, the Japanese submarine I-17 snuck into a channel near Ellwood Oil Field, a large oil well and storage facility outside of Santa Barbara, CA. The submarine surfaced sending explosive shells (a minimum of 12 to as many as 25) from the deck gun then submerged and left to the open ocean. The shelling only caused minor damage to the oil field-but it sparked an invasion panic among Americans. One night after the Elwood attack, the so-called Battle of Los Angeles took place. In response to claimed sightings of "enemy aircraft", American artillery opened fire all across the city, panicking residents. It's still debated if anything actually had been spotted, and if so what it was.