Singapore Prime Minister Lee Kuan Yew made a landmark speech against Malay political dominance in the Malaysian Parliament in Kuala Lumpur in May 1965 which so angered Umno leaders that many felt Singapore had to leave.
Then Malaysian Prime Minister Tunku Abdul Rahman later called the speech the straw that broke the camel's back, while Singapore's former Cabinet minister Lim Kim San called it the speech that changed history.
"I had not expected my speech to play so crucial a part in the Tunku's decision to get Singapore out of Malaysia," Mr Lee wrote in his memoirs.
Before the fateful speech, tension between Mr Lee's People's Action Party (PAP) and Malaysia's ruling Alliance coalition headed by Umno had already been escalating. Mr Lee was making a good impression on the international media.
The PAP had formed the Malaysian Solidarity Convention (MSC), an opposition alliance whose members included parties from Sabah and Sarawak. This did not sit well with some Umno members.
As Mr Lee noted in his memoirs, he made his most important speech in the federal Parliament to "a hostile and tense audience, including a large number of Malay MPs fed daily with anti-PAP, anti-Lee Kuan Yew and anti-Chinese propaganda".
Mr Lee expressed regret that the King's Address at the opening of Parliament "did not reassure the nation that it would continue to progress in accordance with its democratic Constitution towards a Malaysian Malaysia".
The King had also referred to an unspecified threat from within.
Mr Lee noted that he had been called an enemy of Malaysia by some in the Malay press.
"So it is perhaps we - loyal Malaysians gathering to establish a Malaysian Malaysia - who are the threat from within," Mr Lee said. A backbencher from the ruling Alliance coalition shouted, "It is the PAP," The Straits Times reported.
Dr Mahathir Mohamad, then an Umno backbencher and later prime minister of Malaysia, had also attacked the PAP a day before in Parliament, accusing it of being "positively anti-Malay".
Referring to the Chinese in Singapore, Dr Mahathir said: "They have never known Malay rule and couldn't bear the idea that the people they have so long kept under their heels should now be in a position to rule them."
Recalled Mr Lee in his memoirs: "To rule them? I drew a distinction between political equality and the special rights for the economic and social uplift of the Malays."
In Parliament, Mr Lee refuted accusations that the PAP was pro-Chinese, noting that it could not win majority support if it advocated a Chinese Malaysia, as the Chinese made up just 42 per cent of the population.
Speaking in fluent Malay without notes, he laid out a sharp argument against affirmative action, noting that it would not be effective in helping to uplift the livelihoods of poor Malays.
Other races did not oppose Malay rights, he said.
"They, the Malays, have the right as Malaysian citizens to go up to the level of training and education that the more competitive societies, the non-Malay society, has produced. That is what must be done, isn't it?
"Not to feed them with this obscurantist doctrine that all they have got to do is to get Malay rights for a few special Malays and their problem has been resolved."
The speech convinced many in Umno that Mr Lee was a threat. It helped the Tunku make up his mind that Singapore must leave Malaysia.