The Indonesia–Malaysia confrontation or Borneo confrontation (known as Konfrontasi in Indonesia, Malaysia and Singapore) was an armed conflict from 1963 to 1966 that stemmed from Indonesia's opposition to the creation of the state of Malaysia from the Federation of Malaya. After Indonesian president Sukarno was deposed in 1966, the dispute ended peacefully.

The creation of Malaysia was a merger of the Federation of Malaya (now Peninsular Malaysia), Singapore and the British Crown colonies of North Borneo and Sarawak (collectively known as British Borneo, now East Malaysia) in September 1963. Vital precursors to the conflict included Indonesia's policy of confrontation against Dutch New Guinea from March to August 1962 and the Indonesia-backed Brunei revolt in December 1962. Malaysia had direct military support from the United Kingdom, Australia, and New Zealand. Indonesia had indirect support from the USSR and China, thus making it an episode of the Cold War in Asia.

The conflict was an undeclared war with most of the action occurring in the border area between Indonesia and East Malaysia on the island of Borneo (known as Kalimantan in Indonesia). However Indonesia also conducted lower intensity covert actions on the Malay Peninsula and in Singapore. The conflict was characterised by restrained and isolated ground combat, set within tactics of low-level brinkmanship. Combat was usually conducted by company- or platoon-sized operations on either side of the border. Indonesia's campaign of infiltrations into Borneo sought to exploit how ethnically and religiously diverse Sabah and Sarawak were compared to that of Malaya and Singapore, with the intent of unravelling the proposed state of Malaysia.

The jungle terrain of Borneo and the lack of roads straddling the Indonesia–Malaysia border forced both Indonesian and Commonwealth forces to conduct long foot patrols. Both sides relied on light infantry operations and air transport, although Commonwealth forces enjoyed the advantage of better helicopter deployment and resupply to forward operating bases. Rivers were also used as a method of transport and infiltration. Although combat operations were primarily conducted by ground forces, airborne forces played a vital support role and naval forces ensured the security of the sea flanks. The British provided most of the defensive effort, although Malaysian forces steadily increased their contributions, and there were periodic contributions from Australian and New Zealand forces within the combined Far East Strategic Reserve stationed then in Peninsular Malaysia and Singapore.

Initially, Indonesian attacks on East Malaysia relied heavily on local volunteers trained by the Indonesian Army. Over time, the infiltration forces became more organised with the inclusion of a more substantial component of Indonesian forces. To deter and disrupt Indonesia's growing campaign of infiltrations, the British responded in 1964 by launching their own covert operations into Indonesian Kalimantan under the code name Operation Claret. Coinciding with Sukarno announcing a 'year of dangerous living' and the 1964 race riots in Singapore, Indonesia launched an expanded campaign of operations into Peninsular Malaysia on 17 August 1964, albeit without military success. A build-up of Indonesian forces on the Kalimantan border in December 1964 saw the UK commit significant forces from the UK-based Army Strategic Command. Australia and New Zealand deployed roulement combat forces from Peninsular Malaysia to Borneo in 1965–66. The intensity of the conflict began to subside following the coup d'état of October 1965 and Sukarno's loss of power to General Suharto. A round of serious peace negotiations between the two sides began in May 1966, and a final peace agreement was signed on 11 August 1966 with Indonesia formally recognising Malaysia.

Before Konfrontasi, Sukarno sought to develop an independent Indonesian foreign policy, focused on the annexation of Dutch New Guinea as a conclusion of the Indonesian National Revolution, and establishing Indonesia's credentials as a notable international power supporting its own agenda distinct from those of the First and Second World. Indonesia was an important country in developing the Non-Aligned Movement, hosting the Bandung Conference in 1955. Indonesia had relentlessly pursued its claim to Dutch New Guinea from 1950 to 1962, despite facing multiple setbacks in the UN General Assembly in getting its claim recognised by the international community.

Following the Indonesian crisis in 1958, which had included the Permesta rebellion in eastern Indonesia and the declaration of the Revolutionary Government of the Republic of Indonesia, Indonesia had emerged as a notable and rising military power in Southeast Asia. The Central Intelligence Agency (CIA), primarily through its Taiwan-based subsidiary Civil Air Transport (CAT), had been covertly providing support to rebels on remote islands, seeking to weaken, if not outright oust President Sukarno's regime. Beginning in 1957, the CIA was increasing contact with military leaders in Sumatra and Sulawesi critical of the regime. By late 1957, the transport of weapons and ammunition to Sumatra by merchant ships and night submarines had become increasingly common, but the Americans believed that for their clandestine assistance to be truly effective, such operations required the use of British facilities in Singapore to refuel and support CAT missions launched in Bangkok, Taiwan, or the Philippines. With the influx of Soviet arms aid, Indonesia was able to advance its claim to Dutch New Guinea more forcefully. The diplomatic dispute reached its climax in 1962 when Indonesia launched a substantial campaign of airborne and seaborne infiltrations into Dutch New Guinea. While the infiltration forces were soundly defeated by Dutch and Papuan forces, Indonesia was able to lend credence to the threat of an Indonesian invasion of Dutch New Guinea. The Dutch, facing mounting diplomatic pressure from the Indonesians, and also the Americans, who were anxious to keep Indonesia from becoming Communist aligned, yielded and agreed to a diplomatic compromise, allowing the Indonesians to gain control of the territory in exchange for pledging to hold a self-determination plebiscite (the Act of Free Choice) in the territory by 1969. Thus by the close of 1962, Indonesia had achieved a considerable diplomatic victory, which possibly emboldened its self-perception as a notable regional power. It was in the context of this recent diplomatic victory that Indonesia cast its attention to the British proposal for a unified Malaysian state.

Before the British government announced the East of Suez policy in 1968, it had already begun re-evaluating their commitment to maintaining a presence in the Far East as early as the late-1950s. As a part of its withdrawal from its Southeast Asian colonies, the UK moved to combine its colonies in North Borneo with the Federation of Malaya (which had become independent from Britain in 1957), and Singapore (which had become self-governing in 1959). In May 1961, the UK and Malayan governments proposed a larger federation called Malaysia, encompassing the states of Malaya, North Borneo, Sarawak, Brunei, and Singapore. Initially, Indonesia was mildly supportive of the proposed federation, although the Communist Party of Indonesia (PKI) was firmly opposed to it.