An important element behind the Battle of Los Angeles was the submarine I-17. The night before the Battle this Japanese submarine shelled an oil refinery just up the coast, causing little damage but stirring invasion fears. It is critical background for why some anti-aircraft gunners were unusually jumpy the next night.

I-17 had previously been part of an armada of Japanese submarines stationed around Pearl Harbor and then ordered to the West Coast, where she sank the tanker Emidio, the first ship sunk in that operation. She later served on several cargo runs to Guadalcanal, using her pressurized aircraft hangar to transport some of the cargo. She rescued 151 Japanese soldiers and sailors who survived the Battle of the Bismarck Sea, and during the rescue was unsuccessfully attacked by two US PT-boats. She later sank the freighter Stanvac Manila and two of the PT-boats carried aboard as cargo: four other PT-boats floated off the sinking ship and survived. In August 1943 she was ordered to scout US bases in the South Pacific, and after a successful reconnaissance flight by her floatplane on 10 August, she was sunk on 19 August by a New Zealand minesweeper and a group of US floatplanes. HMNZS Tui rescued six survivors, with the remaining 97 crewmen going to the bottom.