'X' was the Japanese seizure of Christmas Island some 220 miles (355 km) to the south of western Java (31 March/3 April 1942).

A British possession measuring 11 miles (17.75 km) by 9 miles (14.5 km) with an area of 52 sq miles (135 km²), the island’s phosphate deposits were worked by a small British management and engineering team as well as some 1,000 Indian, Malay and Chinese labourers, and also had a garrison of 100 British troops.

After the 'J' (ii) conquest of Java in the Netherlands East Indies, Christmas island drew the attention of Imperial General Headquarters. On 7 March 1942 the battleships Haruna and Kongo shelled the island to destroy its commercial installations. The island was then marked for seizure, and a small occupation force departed Makassar, on the island of Celebes taken in 'H' (i), during 25 March.

Japan was interested in the island for its phosphates, and as a possible site for an airstrip and/or emergency supply base.

Soon after the outbreak of hostilities, a naval gun was installed under a British officer, four British non-commissioned officers and 27 Indian soldiers. The first Japanese attack was carried out on 20 January 1942 by the Japanese submarine I-59, which torpedoed a Norwegian freighter, the 4,184-ton Eidsvold, which had been loaded with 3,700 tons of phosphate for delivery to Fremantle in Western Australia. The vessel drifted and eventually sank off West White Beach. Most of the European and Asian staff and their families were evacuated to Perth. Late in February and early in March there were two Japanese air raids. Shelling from a Japanese naval group on 7 March led the district officer to hoist the white flag, but once the Japanese naval group had departed, the British officer lowered the white flag and raised the Union flag once again. During the night of 10/11 March, a mutiny of the Indian troops, abetted by Sikh policemen, led to the killing of the five British soldiers and the imprisonment of the remaining 21 Europeans. At dawn on 31 March, a dozen Japanese bombers struck at the island, destroying the radio station.

For 'X', Rear Admiral Kyuji Kubo’s task force comprised the light cruiser Naka, destroyers Natsugumo and Minegumo, patrol boats PB-34 and PB-36, and two transport vessels (5,193-ton Kimishima Maru and 7,508-ton Kumagawa Maru) carrying 850 men of the 21^st Special Base Force and 24^th Special Base Force and the 102^nd Construction Unit, constituting the Operation X Guard Force. On 29 March this force was supplemented by Rear Admiral Shoji Nishimura’s light cruisers Nagara and Natori, destroyers Amatsukaze and Hatsukaze, and 10,182-ton oiler Akebono Maru, which had departed Bantam Bay on Java.

After the standard pre-landing bombing, on 31 March the occupation force began to bombard the island, and the British garrison surrendered almost immediately. The transports, along with some warships, including Naka, moved in shortly after this and the Japanese quickly took control of the town. The garrison troops were taken prisoner and forced to load the transports with refined phosphate ore, while Japanese troops moved inland to occupy and explore the rest of the island.

Kubo knew there was an Allied submarine in the vicinity, however, for three torpedoes had narrowly missed Naka, his flagship. After an intensive anti-submarine effort, including the use of the cruisers' reconnaissance floatplanes, a large quantity of oil appeared on the water. Seawolf had not been sunk, however, and finally scored a torpedo hit on Naka, amidships on the starboard side, during 1 April, and the warship began to flood. Despite further anti-submarine efforts, Seawolf escaped. After emergency measures, Naka's flooding was contained, and the cruiser was taken in tow by Natori. To help protect the crippled cruiser, the Combined Fleet headquarters despatched the destroyers Fumizuki, Minazuki, Nagatsuki and Satsuki as additional escorts. Naka reached Shonan island near Singapore on 6 April, and two months later, after repairs, departed for Yokosuka. Meanwhile, Natori returned to Christmas Island on 3 April and gathered most of the troops and remaining ships, leaving only 20 men of the 21^st Special Base Force as the garrison.

Sabotaged equipment was repaired and preparations were made to resume the mining and export of phosphate. Isolated acts of sabotage and the torpedoing of the 2,878-ton Nissei Maru at the wharf on 17 November 1942 meant that only small amounts of phosphate were exported to Japan during the occupation. In November 1943, ore than 60% of the island’s population was evacuated to prison camps near Surabaya on Java, leaving a population of just under 500 Chinese and Malays and 15 Japanese to survive as best they could. In October 1945, the British frigate Rother reoccupied Christmas Island.

The island had not been subjected to a full occupation as it was considered unsuitable for an airstrip, so all which the Japanese had gained was the phosphate rock initially loaded on the transports.