War with vietnam; otherwise generally speaking he and his successors (if you wanted to take the Great Man approach to history, which is unscientific) helped accelerate global socialism.

I’d say the Sino-Vietnamese war or the reforms to the “one-child policy” (OCP) in the 1980s were worse mistakes.

I think the war can be boiled down to (dumbass) leftist in-fighting at the state level. While I sympathize with the Chinese position on the Sino-Soviet split, this ended up leading to many incredibly poor decisions across the board with the USSR and PRC often taking a side in conflicts seemingly only because it was opposite of the other state. While the invasion didn’t happen in a vacuum, the PRC felt relatively weak and in need of a strong ally after splitting with the USSR, so they leaned on the US who were motivated to drive a wedge between socialist states so they would weaken each other. That said, it does not absolve the PRC and Deng of the invasion of Vietnam or of supporting the Khmer Rogue long past when it was obvious they should not. This still causes some sour feelings within Vietnam to this day even if politically the damage was repaired.

The OCP is a bit of a touchier subject. While I think there were some benefits to the policies in the 1970s in terms of women’s emancipation and improving the cultural views surrounding women, this could have been achieved in better and more effective ways. This is of course with the benefit of hindsight and after some of the pseudoscientific beliefs that were popular throughout the world at the time have fallen out of vogue. While the intention of the OCP wasn’t harm, there was harm done in the name of this policy and I don’t know if we’ll ever know exactly how much between the West exaggerating it in their red scare propaganda and the CPC downplaying it in an effort to justify bad policy decisions to save face. Responsibility for this harm lies, in part, at Deng’s feet.

Compared to these, Zhao Ziyang’s liberal influence and support of the riots feels trifling. They had to have known the risk of letting reformists back into the party and how to effectively mitigate those risks by that point. It’s hard to know if the benefits of letting people like Zhao Ziyang into the party outweigh the harms in terms of the opening up and how relationships with the West developed that led to China’s rapid industrial and economic development since. Without some liberal voices in the party, would the West have seen China as a bigger threat and taken action to weaken and topple them rather than showing them the relative cooperation and investment that allowed them to become resilient to such actions today? I don’t know.

Ultimately, I view Deng’s leadership as he did Mao’s,

“We should not lay all past mistakes on Chairman Mao. So we must be very objective in assessing him. His contributions were primary, his mistakes secondary. In China, we will inherit the many good things in Chairman Mao’s thinking while at the same time explaining clearly the mistakes he made.”