Margaret K. Lewis is a Professor of Law at Seton Hall University and recent Fulbright Senior Scholar at National Taiwan University. Jeffrey Wasserstrom is Chancellor's Professor of History at UC Irvine and coauthor of the updated third edition of "China in the 21st Century: What Everyone Needs to Know" (Oxford University Press, 2018). The opinions expressed in this commentary are their own.

(CNN) Thirty years ago Monday, the most important Chinese mass movement of the last half-century began when Beijing students gathered to mourn Hu Yaobang, a reformist official.

Soon, massive crowds calling for change were converging on the central plazas of dozens of Chinese cities. On May 20, the government imposed martial law in Beijing, whose Tiananmen Square was the site of the largest rallies. Two weeks later, on June 4, the movement ended after soldiers fired on unarmed civilians on the streets of the capital.

The Chinese Communist Party (CCP), which has ruled the People's Republic of China (PRC) since its founding in 1949, has never allowed an official investigation into the killing. The massacre's death toll remains unknown, but at least several hundred civilians and perhaps ten times that were slain.

Thanks in part to the iconic photo of the " Tank Man " standing up to the armed might of the CCP, June 4 is famous around the world, but discussion of what happened on 6/4 -- known as liusi in Chinese -- remains heavily censored in China and public mourning of the victims is forbidden.

This concerted effort to blot out memory of a 30-year-old event is not unprecedented, and there are parallels in the handling of an earlier massacre across the Taiwan strait. This one, known as 2/28, took place in 1947 in Taipei, the largest city and capital of Taiwan, which is officially known as the Republic of China (ROC).

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