The Australian Campaigns were the Australian military operations against the Japanese forces in New Guinea and other islands of the Bismarck Archipelago, and Borneo during World War II. From October 1944, troops of the First Australian Army began relieving American divisions on Bougainville, New Britain, and the north coast of New Guinea. In New Britain, the Australians conducted a containment operation, and at the end of the war the Japanese garrison at Rabaul was found to number almost 70,000 army and naval personnel. The Australian commander, General Blarney, argued, however, that Australia had a duty to liberate its own territory. Therefore, on Bougainville the 2nd Australian Corps began a slow and careful offensive, which was still proceeding at the end of the war. In New Guinea the 6th Division captured Wewak, driving the Japanese into the mountains.

General MacArthur was at best lukewarm about the justification for these offensives, but he enthusiastically ordered the 1st Australian Corps, under Lieutenant-General Sir Leslie Morshead, to conduct operations in Borneo. The first of these began on May 1, 1945, with the capture of Tarakan after heavy fighting. Next, on June 10 the 9th Australian Division landed on Labuan Island and at Brunei. Blarney was now more wary and he opposed the landing of the 7th Division at Balikpapan. MacArthur warned the Australian government that to cancel the operation would disorganize Allied strategic plans; the government approved the landing. In truth, MacArthur wanted to show the Dutch government that he had attempted to recover part of its territory. The landing on July 1 was the last amphibious operation of the war. In the campaigns of late 1944 and 1945 the Australians lost more than 1,500 killed, but Japan lost ten times that number.

Admiral King had always advocated using American naval power to attack the Japanese in the central Pacific, but MacArthur had argued for resources to enable him to advance through New Guinea toward the Philippines. If both approaches could be sustained, then they would throw the Japanese off-balance, but it was not until the latter months of 1943 that the US navy began to gather the strength necessary to prosecute an island-hopping campaign in the central Pacific. At the time of Pearl Harbor, the US navy had only three carriers in the Pacific; by late 1943 Nimitz had 10 fast large and medium carriers, seven escort carriers, and a dozen battleships. These formed the key elements of the Fifth Fleet under Vice-Admiral Spruance.

Carrier strikes began on June 11, 1944, with troops of the 5th Amphibious Corps under Marine Lieutenant-General Holland Smith landing on Saipan on June 15. Japanese carrier and land-based aircraft attacked the US fleet on June 19, but were totally outclassed by the American aircraft and their more skillful pilots. In the "Great Marianas Turkey Shoot", the Japanese lost 400 aircraft, while the USA lost only 29. During this naval battle three Japanese carriers were sunk by the US aircraft and two by American submarines. Onshore, the Marine and army troops had a savage battle against 32,000 defenders. The Japanese conducted suicide charges, while Japanese civilians leapt to their death from high cliffs as they had been told by their Japanese commanders that the Americans would do terrible things to the civilian populations. By July 9, after 23 blood-drenched days of ferocious fighting, the US Marines had secured Saipan. Total Japanese deaths numbered 30,000. Meanwhile, US Marine and army troops captured Guam and Tinian. The defeat of the Japanese carrier force and the seizure of the Marianas were a devastating blow to the Japanese high command. On July 18, Tojo resigned as Prime Minister and War Minister, succeeding him Lieutenant-General Koiso Kuniaki.

The Aleutian Islands Campaign was a US military campaign carried out against the Japanese Army on the Aleutian Islands from early May to July 29, 1943. The ground battle took place on the island of Attu between the US 7th Infantry Division and Japanese Army units. The result was a US victory.

In a daring operation, on the night of July 28/29, 1943, under cover of fog, the Japanese navy evacuated its garrison of more than 5,000 troops from Kiska Island. The 34,000 American and Canadian troops who landed there on August 15 took several days to discover that they faced no opposition. For the Japanese, the Aleutian Islands campaign had been a disastrous waste of men and materiel when they had been under increasing pressure in the south and south-west Pacific.

On December 8, 1942, one day after the attack on Pearl Harbor, the Japanese began the invasion of the Philippines, with the main landing taking place on December 22, 1941. The invasion of the Philippines was carried out by Lieutenant-General Homma Masaharu’s Fourteenth Army, at Lingayen Gulf on Luzon. By December 1941, the combined defense forces in the Philippines were organized into the US Army Forces in the Far East (USAFFE), which included the Philippine Army’s 1st Regular Division, 2nd (Constabulary) Division, and 10 mobilized reserve divisions, and the United States Army’s Philippine Department. However, these troops were inexperienced and lack enough training. After the initial engagements, the Japanese inflicted severe damage on the local forces, which began to rapidly fall back.

Seeing the victory in sight, the Japanese high command advanced by a month their timetable of operations in Borneo and Indonesia, and withdrew their best troops and the bulk of their airpower from the Philippines in early January 1942. Nevertheless, the defenders held defensive positions in the Bataan Peninsula, fighting against the Japanese for four more months.

Between December 1941 and March 1942, Japanese forces conducted a series of successful military campaigns. Perhaps the most remarkable campaign was the invasion of Malaya. It began on December 8, 1941, with the landing in north-east Malaya of troops from the Japanese Twenty-Fifth Army, under Lieutenant-General Yamashita Tomoyuki. Yamashita’s force of 60,000 men was opposed by 88,000 British, Australian, Indian, and Malayan troops, commanded by Lieutenant-General Arthur Percival, but the Japanese naval and land-based aircraft completely outnumbered and outclassed the British air force. On December 10, the British suffered a devastating blow when Japanese aircraft sank the battleship Prince of Wales and the battle cruiser Repulse in the South China Sea.

Advancing more than 600 miles (1,000km), by January 31, 1942, the Japanese Army had pushed the Commonwealth forces back to Singapore. Although they had suffered heavily, the Commonwealth forces had, however, been reinforced and now numbered 88,000. Yamashita attacked with 60,000 troops, crossing the Johore Strait on the night of February 7/8. On February 15, Percival surrendered his force. This defeat in the hands of the Japanese was Described by Winston Churchill as the "worst disaster in British military history," as the fall of Singapore had shattered British prestige in the Far East. Elsewhere, the Japanese were conducting similar military campaigns. Only America’s entry into World War II would stop the Japanese conquest in Asia and the Pacific.

Map of Japanese Invasion of Malaya