Mahathir had been politically active since the end of the Japanese occupation of Malaya, when he joined protests against the granting of citizenship to non-Malays under the short-lived Malayan Union. He later argued for affirmative action for Malays at medical college. While at college he contributed to The Straits Times under the pseudonym "C.H.E. Det", and a student journal, in which he fiercely promoted Malay rights, such as restoring Malay as an official language. While practising as a doctor in Alor Setar, Mahathir became active in UMNO; by the time of the first general election for the independent state of Malaya in 1959, he was the chairman of the party in Kedah. Despite his prominence in UMNO, Mahathir was not a candidate in the 1959 election, ruling himself out following a disagreement with then Prime Minister Tunku Abdul Rahman. The relationship between the two Kedahans had been strained since Mahathir had criticised Abdul Rahman's agreement to the retention of British and Commonwealth forces in Malaya after independence. Now Abdul Rahman opposed Mahathir's plans to introduce minimum educational qualifications for UMNO candidates. For Mahathir this was a significant enough slight to delay his entry into national politics in protest. The delay did not last for long. In the following general election in 1964, he was elected as the federal parliamentarian for the Alor Setar-based seat of Kota Setar Selatan.Elected to parliament in a volatile political period, Mahathir, as a government backbencher, launched himself into the main conflict of the day: the future of Singapore, with its large and economically powerful ethnic Chinese population, as a state of Malaysia. He vociferously attacked Singapore's dominant People's Action Party for being "pro-Chinese" and "anti-Malay" and called its leader, Lee Kuan Yew, "arrogant". Singapore was expelled from Malaysia in Mahathir's first full year in parliament. However, despite Mahathir's prominence as a backbencher, he lost his seat in the 1969 election, defeated by Yusof Rawa of the Pan-Malaysian Islamic Party (PAS). Mahathir attributed the loss of his seat to ethnic Chinese voters switching support from UMNO to PAS (being a Malay-dominated seat, only the two major Malay parties fielded candidates, leaving Chinese voters to choose between the Malay-centric UMNO and the Islamist PAS).Large government losses in the election were followed by the race riots of 13 May 1969, in which hundreds of people were killed in clashes between Malays and Chinese. The previous year, Mahathir had predicted the outbreak of racial hostility. Now, outside parliament, he openly criticised the government, sending a letter to Abdul Rahman in which the prime minister was criticised for failing to uphold Malay interests. The letter, which soon became public, called for Abdul Rahman's resignation. By the end of the year, Mahathir had been fired from UMNO's Supreme Council and expelled from the party; Abdul Rahman had to be persuaded not to have him arrested.While in the political wilderness, Mahathir wrote his first book, The Malay Dilemma, in which he set out his vision for the Malay community. The book argued that a balance had to be achieved between enough government support for Malays so that their economic interests would not be dominated by the Chinese, and exposing Malays to sufficient competition to ensure that over time, Malays would lose what Mahathir saw as the characteristics of avoiding hard work and failing to "appreciate the real value of money and property". The book continued Mahathir's criticism of Abdul Rahman's government, and it was promptly banned.  The ban was only lifted after Mahathir became prime minister in 1981; he thus served as a minister and deputy prime minister while being the author of a banned book. Academics R. S. Milne and Diane K. Mauzy argue that Mahathir's relentless attacks were the principal cause of Abdul Rahman's downfall and subsequent resignation as prime ministe

Question: Who was mainly responsible for the resignation of prime minister according to Diane?
Answer: Mahathir