'Horror' was the US geographical rather than operational codename for Ulithi atoll at the western end of the Caroline islands group in the western Pacific (1941/45).

Ulithi atoll lies 370 miles (595 km) to the to the north-east of the Palau islands group, the same distance to the south-west of Guam in the Mariana islands group, and 118 miles (190 km) to the east-north-east of Yap in the Caroline islands group. Less than 40 miles (65 km) to the east of Ulithi is small Fais atoll.

Ulithi is an irregularly shaped atoll some 22 miles (35.5 km) long on its north/south axis and 14 miles (22.5 km) wide across its broader northern portion. The atoll comprises 40 islets, with a total land area of 1.7 sq miles (4.5 km²), surrounding a lagoon about 22 miles (36 km) long and up to 15 miles (24 km) wide with an area of 212 sq miles (548 km²). The rim reef is broken in many places, but the main passages into the lagoon are Mugai and Dowarugui Channels on the upper east side. Small islands and islets are scattered around the rim, with most of the larger ones on the northern side, though others are concentrated on the upper western and the lower eastern sides: the largest of the islands is Falalop ('Ameliorate') in the atoll’s north-eastern area. Running round the north side of the rim from Falalop, and enclosing Ulithi anchorage, which is capable of accommodating as many as 700 ships, are nearby Asor ('Identical'), Sorlen ('Keenset'), Mogmog ('Lith­arge'), Potangeras ('Acetylene') and Lam (no codename) among a cluster of islets and reefs. On the southern side of the Mugai Channel is Mangejang ('Amazement') and spread out to its south are Fassarai ('Aggravate'), Lossau, Feitabul and Pugeluc. A few islets are located to the east of Ulithi on the Zohhiiyonu Bank. All these islets are low and flat, and are scattered with palms and brush. The climate is hot and humid, and the area experiences heavy rain fall.

Germany bought the island group from Spain in 1899, but in 1914 the Japanese occupied it and in 1920 received a League of Nations mandate to administer the group. The Japanese had established a weather and radio station in the atoll, occasionally used the lagoon as a fleet anchorage, and had begun the construction of an airfield and seaplane base on Falalop, but these were abandoned by a time late in 1944, and most of the native population was evacuated to Yap.

Admiral Chester W. Nimitz, commander-in-chief of the Pacific Fleet and the Pacific Ocean Areas, was a long-time map enthusiast, noticed the atoll, realised its strategic potential and arranged for its seizure on 23 September 1944 during the Palau islands campaign.

After it had been decided to cancel the planned 5 October descent on the strongly held Yap atoll by Major General John R. Hodge’s XXIV Corps in favour of an assault directly on Ulithi' planned 5 October assault on strongly held Yap, the assault on Ulithi was delayed as the forces tasked with that mission were reassigned. On 16 September it was decided to proceed with the plan to seize Ulithi, and the only unit available to the Western Landing Force and Troops was the 323^rd Infantry of Major General Paul J. Mueller’s 81^st Infantry Division, and the assault was set for 22 September.

At this time there was still unwarranted optimism about the progress of 'Stalemate II' on Peleliu, and Colonel Watson’s 323^rd Regimental Combat Team was detailed for the occupation with cover provided by Rear Admiral William H. P. Blandy’s task group.

The Ulithi Attack Force (Task Group 33.1) departed the Palau islands group on 21 September and arrived off Ulithi on the following day. A preliminary reconnaissance by a party which had landed on 21 September had meanwhile found no Japanese. The unloading of the main force occupied two days, and after the main landing force had occupied Falalop, Asor, Mogmog and Potangeras the islands were declared secure. Elements of the 323^rd Infantry then proceeded to search Ngulu atoll between the Palau islands group and Yap atoll on 16/17 October, Pulo Anna island to the south of the Palau islands group on 20 November, Kayangel atoll off the northern end of the Palau islands group on 28 November/1 December, and Fais island to the south-east of Ulithi on 1/4 January 1945. On this last the US force discovered 17 Japanese naval personnel and civilian employees, of whom eight were killed, six were taken prisoner and three, it is believed, fled by boat. The US losses were two men killed and five wounded.

The Advanced Fleet Anchorage, Ulithi Atoll was soon established and a new airfield, with a 3,500-ft (1065-m) runway, and a seaplane base were built on Falalop, while other support facilities, including a fleet recreation area, were constructed on Asor, Sorlen, Mogmog and Potangeras, the last three also gaining small landing strips for local flights by light aircraft.ological station remained in oper­ation there until 1948.