'Ultimatum' was the US geographical rather than operational codename for Pagan island in the Mariana islands group of the central Pacific (1941/45).
Pagan lies about 180 miles (290 km) to the north of Saipan island, and to its south is the tiny island of Alamagan and to its north the island of Agrihan. Pagan in thus the fifth island from the northernmost in the chain. The island is 10.1 miles (16.25 km) long on its north-east/south-west axis and 3.73 miles (6 km) wide with an area of 18.24 sq miles (47.23 km˛), and comprises two lobes, the smaller of them in the south, connected by a strip of land only 650 yards (595 m) wide. Despite the fact that it possesses three active volcanoes reaching a height of 1,870 ft (570 m) at Mt Pagan on the northern lobe and two more on the southern lobe, these latter reaching heights of 1,883 and 1,798 ft (574 and 548 m), the island was populated: in 1942 the Japanese civilian population was 413 persons, and there were also 229 Chamorro residents. A narrow plain is situated on the island’s central neck, but the island is divided by a series of cross-island cliffs. The coast is rocky and lined with cliffs, and the only landing sites are at Apaan Bay on the west coast, where the main settlement, Shomoshun, is located, and an area on the eastern side.
The Japanese had constructed an airfield on the northern end of the narrow neck before the US 'Forager' descent on the Mariana islands group, and in May 1944 3,500 men of the 9th Independent Mixed Regiment reached Pagan island from Japan as the core of the garrison, together with 800 naval (main air service) personnel and 1,000 labourers. The regiment was commanded by Colonel Umehachi Amau, who became the senior military commander remaining in the Mariana islands group after the fall of Guam.
US air attacks on Rota and Pagan islands began on 11 June 1944 and continued intermittently until the war’s end as both islands were bypassed by US forces. Isolated by the continued Allied offensive and receiving supplies only occasionally by submarine, the garrison of Pagan soon faced starvation, and several hundred men died of malnutrition before the surrender of Japan. The Pagan island garrison surrendered on 2 September 1945 to representatives of the US Army as the same time as Rota island surrendered to representatives of the US Marine Corps.