'Hikari' was a Japanese unrealised plan for an aerial reconnaissance of Ulithi atoll in the Caroline islands group (August 1945).

The object of the operation was the provision of targeting information in support of the planned 'Arashi'. The task was entrusted to four Nakajima C6N1 Saiun floatplanes that were to be delivered to Truk atoll, in another part of the Caroline islands group and still held by the Japanese after being bypassed by the forces of Admiral Chester W. Nimitz’s Pacific Fleet, by two submarines attached to Captain Ryunosuke Ariizumi’s 1^st Submarine Flotilla of Vice Admiral Chuichi Hara’s 4^th Fleet.

I-13 was intercepted and sunk on it way to Truk, but I-14 reached the atoll and unloaded its two aircraft, which had been assembled before the cancellation of 'Arashi'.

Germany had bought the Caroline islands group from Spain in 1898, and the islands were occupied by Japan in 1914 soon after the start of World War I. After this war’s end, Japan was awarded a League of Nations mandate to administer the islands which, in defiance of a clause in the Washington Naval Treaty of 1922, it then fortified and developed as a base area.

As World War II’s Pacific campaign advanced to the west, the US Navy required a forward base for the support of its continuing operations. The Japanese had established a radio and weather station on Ulithi atoll, and earlier in the war had made occasional use of the atoll’s large lagoon as an anchorage and seaplane base, but had abandoned it by 1944. Ulithi was perfectly positioned to act as a staging area for the US Navy’s operations in the western Pacific.

The most westerly of the Caroline islands group, Ulithi lies 360 miles (580 km) to the south-west of Guam in the Mariana islands group, 850 miles (1370 km) to the east of the Philippine islands group, and 1,300 miles (2100 km) to the south of Tokyo. Ulithi’s 40 small islands are all very low, but the reef measures approximately 22 by 14 miles (34.5 by 22.5 km) and encloses a huge anchorage with an average depth of 80 to 100 ft (24.5 to 30.5 m). While the anchorage was well situated, there were at the time no port facilities to repair ships or resupply the fleet.

The Ulithi Attack Force (Task Group 33.1) departed the Palau islands group on 21 September 1944 to reach Ulithi on the following day, but on 21 September a reconnaissance party had discovered Ulithi to be unoccupied by the Japanese, and on 23 September elements of the 323^rd Regimental Combat Team of Major General Paul G. Mueller’s 81^st Division landed unopposed to take possession of Falalop, Asor, Mogmog and Potangeras islets round the north of the lagoon, and a few days later a battalion of 'Seabee' naval construction personnel arrived.

The survey ship Sumner surveyed the lagoon and reported it capable of holding 700 vessels, and Ulithi then became the Pacific base for the major operations of the time late in the war, including the 'King II' and 'Iceberg' operations against the islands of Leyte and Okinawa. The lagoon’s huge anchorage capacity was greater than that of either Majuro atoll in the Marshall islands, or Pearl Harbor. The US Navy transferred the local islanders to another of the islets for the rest of the war.

Next to arrive at Ulithi was what Nimitz called his 'secret weapon', namely Service Squadron 10 under the command of Commodore Worrall R. Carter, who had devised this magnificent mobile service force that made it possible for the US Navy to have repair facilities and resupply facilities thousands of miles away from any permanent naval port. Service Squadron 10 now converted the lagoon into a serviceable naval station. Pontoon piers of a new design were built at Ulithi, each consisting of pontoon sections filled with sand and gravel, and then sunk. The pontoons were anchored in place by guy ropes to deadmen on shore, and by iron rods driven into the coral. Connecting tie pieces ran across the tops of the pontoons to hold them together into a pier. Despite extremely heavy weather on several occasions these pontoon piers stood up remarkably well. They gave extensive service, with little requirement for repairs. Piers of this type were also installed by the 51^st Battalion to be used as aviation fuel mooring piers near the main airfield on Falalop.

Within a month of the occupation, a whole floating base was in operation at Ulithi. Some 6,000 ship fitters, artificers, welders, carpenters and electricians arrived aboard repair ships, destroyer tenders, and floating dry docks. Ajax had an air-conditioned optical shop and a metal fabrication shop with a supply of base metals from which she could make the specific alloy to form any part needed. Looking like a large tanker, Abatan distilled sea water into drinking water and baked bread and pies. A special barge produced 500 US gal (416.3 Imp gal; 1893 litres) of ice cream per shift. The dry docks towed to Ulithi were large enough to lift dry a 45,000-ton battleship.

Fleet oilers sortied from Ulithi to meet the task forces at sea, refuelling the warships a short distance from their combat operational areas. The result was something never seen before: a vast floating service station enabling the entire 3^rd and 5^th Fleets to operate indefinitely at unprecedented distances from their mainland bases.

The Japanese had considered that the vastness of the Pacific Ocean would make it very difficult for the US to sustain operations in the western Pacific, but with the Ulithi naval base to refit, repair and resupply, many ships were able to deploy and operate in the western part of the Pacific for a year or more without returning to the naval base at Pearl Harbor.

The Japanese had built an airstrip on Falalop, and this was expanded and resurfaced, the runway running the full width of the island and extending at its eastern end some 20 ft (6.1 m) past the natural shoreline. A number of small airstrips for light aircraft were also built on several of the smaller islands.

The 'Seabees' completed a fleet recreation centre on Mogmog island to accommodate 8,000 men and 1,000 officers daily, and a 1,200-seat theatre was completed in 20 days. At the same time, a 500-seat chapel was built. A number of the larger islands were used as additional recreational facilities and also as bases to support naval vessels and facilities within the lagoon.

The Japanese still held Yap, some 95 miles (150 km) to the south-west of Ulithi, and early in the US occupation of Ulithi launched a number of attacks that caused no damage to the 'Seabee' teams working on the islands.

On 20 November 1944 the Ulithi anchorage was attacked by Japanese kaiten human torpedoes launched from two submarines. The destroyer Case rammed one of these during the early hours of the morning, but at 05.47 the anchored fleet oiler Mississinewa was struck and sunk. Destroyers began dropping depth charges throughout the anchorage. After the war Japanese naval officers said that two tender submarines, each carrying four manned torpedoes, had been sent to attack the fleet at Ulithi. Three of the suicide torpedoes could not be launched as a result of mechanical problems, and another ran aground on the reef. Thus it was only two which managed to enter the anchorage.

A second kaiten attack in January 1945 was defeated when the submarine I-48 was sunk by the destroyer escort Conklin with the loss of all 122 men on board. On 11 March 1945 two long-range aircraft flying from Japan made a nocturnal kamikaze attack in 'Tan No. 2'. One struck the 'Essex' class fleet carrier Randolph which, despite the black-out, had an illuminated cargo: the aeroplane struck over the stern starboard quarter, damaging the flight deck and killing a number of crewmen; and the other mistook the baseball field on Mogmog for a carrier.

By 13 March 1945 there were 647 ships at anchor at Ulithi, and with the arrival of amphibious forces staging for the 'Iceberg' invasion of Okinawa the number of ships at anchor peaked at 722. After Leyte had been secured, the Pacific Fleet moved its forward staging area to Leyte, and Ulithi was all but abandoned.