Ngo Dinh Diem was the founder and first president of the Republic of Vietnam (1955-1963).
Ngo* was a member of the Ngo Dinh family, aristocratic Roman Catholics that had close ties to Bao Dai, the last Emperor of Annam. Although he held a post in the colonial government of French Indochina, he was opposed to colonial rule. He was also opposed to communism, and declined an offer from Ho Chi Minh to serve in the communist nationalist government at the end of the war against the French.
He had spent the war years traveling abroad and making influential friends, particularly in the United States, hoping to position himself as an anti-communist political leader in the post-war government. He got his start when he was named Prime Minister of the State of Vietnam by Bao Dai in 1954. Although that nascent office held little real power, Ngo quickly developed a power base, leveraging his family and religion, and the support from the government of the United States, which saw him as a pillar of anti-communism at a time when anti-communism was a paramount characteristic for foreign leaders who were to be favored by the US.
By mid-1955, he had gained control over most of southern Vietnam and, with US support, was strong enough to thwart the Geneva Accord of 1954 by declaring his government to be the new Republic of Vietnam and refusing to conduct the national election that was mandated by the Geneva Conference. He held his own election for the south without the international observation required by the Geneva Accords, excluding the Vietminh altogether. That he won the rigged election with 98% of the vote was no surprise to anyone.
In the seven years that followed, Ngo ran a nepotistic, corrupt and brutal government that strongly favored his family and the Roman Catholic minority of Vietnamese society. The rest of the public, which was Buddhist, generally despised the government and their hearts and minds were ripe for insurrection. The steadily worsening situation culminated in the Buddhist Crisis of 1963. By mid-1963, with Buddhist monks burning themselves alive in public to protest against Diem and with increasingly effective attacks by the communist forces, the US government finally woke up to reality and decided that Diem had to go. A few months later, Ngo Dinh Diem and his brother Ngo Dinh Nhu were shot to death in a coup led by a group of ARVN generals.
*Vietnamese names are like Chinese names and Japanese names in that the surname comes first, followed by the personal name. Many important Vietnamese persons of the Vietnam war era are mistakenly (and rather rudely) referred to in the Western press and in publications by the last part of their personal names. Thus, Vo Nguyen Giap is usually referred to as "Giap", Ngo Dinh Diem as "Diem", Nguyen Cao Ky as "Ky", and so on. A major exception is Ho Chi Minh, whose name is usually (and correctly) shortened to "Ho" rather than "Minh".