It started by Datuk Onn Jaafar (Father of the 3rd Malaysia Prime Minister: Datuk Hussein Onn)).. due to Malayan Union which was formed after British returned to Malaya when Japan surrendered.
He had formed United Malays National Organizations in order to against Malayan Union to be formed. Coz the Malay and Sultans special exemptions and rights will be eliminated.
Due to some conflict within the UMNO, Datuk Onn released his post and Tunku Abdul Rahman had become the President. During the period where
Tunku tried to gain independence from British.. Alliance among UMNO, MCA and MIC had been made in order to obtain independence and majority supports frm people in Malaya.
Throughout this period, the Malayan Emergency had been on-going. The Malayan Races Liberation Army (MRLA), supported by the Malayan Communist Party (MCP), committed acts of terror such as tearing down farms, disrupting transportation and communication networks, attacking police stations, and so forth. Their stated goal was the end of colonialism in Malaya. The British declared the MCP, along with several left-wing political groups, illegal in 1948.
In 1956, the Tunku led a group of negotiators, comprising Alliance politicians and representatives of the Malay rulers, to London. There, they brokered a deal with the British for independence. The date of independence was set as August 31, 1957, on the condition that an independent commission be set up to draft a constitution for the country.
Source:http://en.wikipedia.org/wiki/United_Malays_National_Organisation