II January–May 1945 Upon commissioning, I-400 was attached to the Kure Naval District and assigned to Submarine Division 1 in the 6th Fleet. She also was assigned to Submarine Squadron 11 for workups. She got underway from Sasebo on the day of her commissioning to begin workups in the western Seto Inland Sea with the submarine , joined in January 1945 by I-400′s sister ship . After the United States Army Air Forces conducted a major fire-bombing raid on Tokyo on the night of 9–10 March 1945, the 6th Fleet proposed to the Imperial Japanese Navy General Staff a retaliatory raid on San Francisco, California, by Aichi M6A1 Seiran floatplane bombers launched by the submarines of Submarine Division 1 , but by April 1945 the staff's vice chief, Vice Admiral Jisaburō Ozawa, had rejected the proposal. was in drydock at Kure, Japan, on 19 March 1945 when the United States Navy′s Task Force 58 launched the first Allied air strike against the Kure Naval Arsenal. More than 240 aircraft from the aircraft carriers , , , , , , and attacked Japanese ships in the harbor at Kure. American aircraft strafed I-400, which returned fire with her Type 96 25 mm antiaircraft guns, and one of her gunners was killed. I-400 visited Moji, Japan, from 5 to 6 April 1945, then returned to Kure. On 14 April 1945, she got underway from Kure with the commander of Submarine Division 1, Captain Tatsunosuke Ariizumi, embarked to make a voyage to Dairen, Manchukuo, to pick up a cargo of fuel oil. She called at Dairen from 20 to 23 April 1945 and loaded 1,700 tons of fuel oil, then headed back to Kure, which she reached on 27 April 1945. While she was at Kure, shipyard workers installed a snorkel aboard her during May 1945. Panama Canal operation By 1 June 1945, all four submarines of Submarine Division 1—I-13, , I-400, and I-401—had been fueled and equipped with snorkels. I-400 got underway from Kure on 2 June 1945 for a voyage via the Shimonoseki Strait, the Tsushima Strait, and the Sea of Japan to Nanao Bay on the western coast of Honshu near Takaoka, Japan. On 5 June 1945, she arrived in Nanao Bay, where all four submarines gathered and were joined by six Aichi M6A1 Seiran ("Clear Sky Storm") aircraft of the Kure-based 631st Naval Air Group, which flew in after