World War II: The Battle of Rabaul commences Japan's invasion of Australia's Territory of New Guinea.

World War II or the Second World War was a global conflict between two coalitions: the Allies and the Axis powers. Nearly all of the world's countries participated, with many nations mobilising all resources in pursuit of total war. Tanks and aircraft played major roles, enabling the strategic bombing of cities and delivery of the first and only nuclear weapons ever used in war. World War II is the deadliest conflict in history, causing the death of 70 to 85 million people, more than half of whom were civilians. Millions died in genocides, including the Holocaust, and by massacres, starvation, and disease. After the Allied victory, Germany, Austria, Japan, and Korea were occupied, and German and Japanese leaders were tried for war crimes.

The Battle of Rabaul, also known by the Japanese as Operation R, an instigating action of the New Guinea campaign, was fought on the island of New Britain in the Australian Territory of New Guinea, from 23 January into February 1942. It was a strategically significant defeat of Allied forces by Japan in the Pacific campaign of World War II, with the Japanese invasion force quickly overwhelming the small Australian garrison, the majority of which was either killed or captured. Hostilities on the neighbouring island of New Ireland are usually considered to be part of the same battle. Rabaul was significant because of its proximity to the Japanese territory of the Caroline Islands, site of a major Imperial Japanese Navy base on Truk.

The New Guinea campaign of the Pacific War lasted from January 1942 until the end of the war in August 1945. During the initial phase in early 1942, the Empire of Japan invaded the Territory of New Guinea on 23 January and Territory of Papua on 21 July and overran western New Guinea beginning on 29 March. During the second phase, lasting from late 1942 until the Japanese surrender, the Allies—consisting primarily of Australian forces—cleared the Japanese first from Papua, then New Guinea, and finally from the Dutch colony.