At the Politburo Standing Committee meeting on May 17, Zhao Ziyang's attitude toward martial law was contrary to that of Li, Deng, and the others. Later, in his book "Prisoner of the State: The Secret Journal of Premier Zhao Ziyang", he described how he felt: "My mood at the time was very uneasy. I was thinking to myself: No matter what, you can't let yourself be General Secretary of a party that uses the army to suppress students."

After the meeting ended, Zhao Ziyang angrily asked Bao Tong to draft him a letter of resignation for the Standing Committee and Deng Xiaoping. The full text read: "I have considered this for a long time. Because of my level of knowledge and ideology, I have no way to implement the policy by Comrade Xiaoping and the Standing Committee meeting on imposing martial law in Beijing. I continue to retain my original opinion. To this end, I ask to resign as general secretary of the CCP Central Committee and first vice-chairman of the Central Military Commission."

On the 18th, when Yang Shangkun first learned that Zhao Ziyang was about to resign, he immediately called Zhao Ziyang to persuade him to change his mind and not escalate the situation. After repeated persuasion by Yang Shangkun, Zhao Ziyang agreed not to send out the resignation letter. That night, the Central Committee Politburo Standing Committee held a more detailed discussion on martial law measures. Zhao then made his final appearance at the meeting, and said to the Standing Committee: "It seems that my historical mission has been completed."

References: "The Critical Moment – Li Peng Diaries"; "The Tiananmen Papers"; "Prisoner of the State: The Secret Journal of Premier Zhao Ziyang"