In 1961, Borneo was divided into four administrations. Kalimantan, a province of Indonesia, is located in the south of Borneo. To the north is the Kingdom of Brunei and two British colonies; Sarawak and Britain North Borneo, later called Sabah. As part of its withdrawal from its Southeast Asian colonies, Britain tried to combine its colonies in Borneo with the Malay Peninsula to form Malaysia.
This plan was opposed by the Indonesian Government; President Soekarno argued that Malaysia was only a British puppet, and the consolidation of Malaysia would only add British control in the region, thus threatening Indonesia’s independence. The Philippines also makes claims over Sabah, arguing the area has historical links with the Philippines through the Sulu Islands.
In Brunei, the Tentara Nasional Kalimantan Utara (TNKU) rebelled on December 8, 1962. They tried to capture the Sultan of Brunei, the oil fields and the European hostages. Sultan escapes and asks for British help. He received British and Gurkha troops from Singapore. On December 16, the British Far Eastern Command claimed that all major rebel centers had been overcome, and on April 17, 1963, the leader of the rebellion was arrested and the uprising ended.
January 1963, Indonesian Foreign Minister Soebandrio announces a hostile attitude towards Malaysia. The Konfrontasi policy launched, it was an “undeclared war” fought between Malaysia and Indonesia. The conflict started when Malaysia, consisting of Malaya, Sabah, Sarawak, and Singapore, was formed on September 16, 1963. Indonesia under President Sukarno was vehemently against the formation of Malaysia, which Sukarno saw as a British strategy to contain Indonesia’s geopolitical ambitions in the region (the Philippines was also against the formation of Malaysia but apart from breaking off diplomatic relations did not resort to the use of military means). Sukarno launched a “Ganyang Malaysia” or “crush Malaysia” campaign, initially using political, economic, and propaganda means. When these actions failed, he decided to launch military attacks against Malaysia.
May 1964, Indonesia started military attack to Malaysia, then officially the war between Indonesia and Malaysia started. Eventually the war will led to form ASEAN, a partnership regional organisation between south east Asia country, including Indonesia and Malaysia inside.