'K-1' was a Japanese naval reconnaissance of and limited attack on Pearl Harbor, the main base of Admiral Chester W. Nimitz’s US Pacific Fleet in the Hawaiian islands group (4 March 1942).
The attack was carried out by two Kawanishi H8K 'Emily' flying boats lifting off from Wotje atoll in the Marshall islands group, refuelled at French Frigate Shoals, 530 miles (850 km) to the north-west of the island of Oahu, from the ocean-going submarines I-15, I-19 and I-26, and using the submarines I-9 as a radio navigation beacon between between Wotje and the French Frigate Shoals and I-23 as a weather reporter and rescue boat to the south of the Hawaiian islands group.
The primary object of the operation was to provide Admiral Isoroku Yamamoto’s Combined Fleet with accurate and timely intelligence of the Pacific Fleet’s strength and movements, and most specifically to assesses the extent to which repairs of the damage inflicted in 'Ai' had been completed and also to bomb the vital repair dock.
The planning for 'K-1' was started in the weeks immediately after 'Ai' as the Imperial Japanese naval air force considered ways to exploit the excellent payload/range performance of its new H8K flying boat. Wholly unrealistic plans for the bombing of targets in California and even Texas were discussed, but a more realistic scheme was then developed in response to the perceived need for up-to-the-minute information about the repairs being effected on the damaged US Navy facilities at Pearl Harbor on Oahu island. An assessment of the repairs to the docks, yards and airfields of Oahu would help Japanese naval planners to determine the extent of the US ability to project naval power in the months to come.
The initial plan called for the use of five H8K boats. These were to fly to the French Frigate Shoals, the largest atoll at the north-western end of the Hawaiian islands chain, to be refuelled by submarines and then lift off once more for their attack on Oahu. If the first raid was successful, the Japanese believed, additional raids would be made. In a repeat of events just before 'Ai', US codebreakers were able to provide a warning that the Japanese were preparing reconnaissance and disruption raids, using flying boats which would refuel at French Frigate Shoals, but once again were largely ignored by their superiors.
In the event only two H8K flying boats were available with crews led by Lieutenant Hisao Hashizume and Ensign Shosuke Sasao. The boats were despatched to Wotje atoll in the Marshall islands group, and here each was loaded with four 551-lb (250-kg) bombs. From Wotje the boats flew 1,900 miles (3060 km) to French Frigate Shoals to refuel, then set off for Oahu island. In addition to reconnaissance, the boats were to bomb the 'Ten-Ten' dock (so-named for its length of 1,010 ft/308 m) at the Pearl Harbor naval base in an effort to disrupt salvage and repair efforts. The submarine I-23 was to station itself just to the south of Oahu as a planeguard and weather spotter for the boats, but was lost some time after 14 February.
The US radar stations on Kauai and later on Oahu detected and tracked the two boats as they approached the main body of the Hawaiian islands group, prompting a search by Curtiss P-40 fighters. Consolidated PBY flying boats were also launched to look for the aircraft carriers that were assumed to have launched the two Japanese aircraft. In the event none of the US aircraft spotted the Japanese boats, which were flying at an altitude of 14,765 ft (4500 m) as the result of the presence of clouds. These clouds also confused the Japanese pilots, however.
Using the Kaena Point lighthouse for a position fix, Hashizume decided to attack from the north, but Sasao did not hear Hashizume’s instruction and turned to skirt the south coast of Oahu. Having lost sight of his wingman and only able to see small patches of the island, Hashizume dropped his four bombs on the slopes of Tantalus Peak, an extinct volcano cinder cone just to the north of Honolulu, during the early hours of the morning. The bombs impacted about 1,000 ft (305 m) from Roosevelt High School, blasting craters 6 to 10 ft (1.8 to 3 m) deep and 20 to 30 ft (6.1 to 9.1 m) wide, shattering windows but causing no other damage. It is assumed that the bombs dropped by Sasao’s boat fell into the sea off the coast of Waianae or near the sea approach to Pearl Harbor.
The two boats then flew off to the south-east and reached Jaluit atoll, also in the Marshall islands group.