Editor's Note: (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).

For decades, the Nationalist Party (KMT) whose soldiers carried out the 2/28 massacre prevented official investigation of the bloodshed. The size of the death toll thus remains uncertain, though it is believed to be between several thousand and 25,000.

In 1977, 30 years after 2/28, the KMT continued to ban all discussion of the event.

Two governments, two massacres

In 1977, Taiwan was still, like today's PRC, under one-party authoritarian rule.

A key reason the memories of the 1947 massacre threatened the KMT in 1977 and memories of the 1989 massacre threaten the CCP now is that, in each killing, soldiers touted as benevolent defenders of the people behaved like brutal invaders.

Today however, Taiwan is a democracy, and 2/28 is marked nationwide as Peace Memorial Day. What can we learn from the similarities between the massacres -- and that the KMT eventually apologized for 2/28?

After Japan's 1945 surrender in World War II, Chiang Kai-shek's forces seized Taiwan and claimed it for the ROC, which then included much of the territory of today's PRC. Heavy-handed efforts to subdue the island by Chiang's KMT were, not surprisingly, met with resistance. Tensions flared on February 27, 1947, when police struck a widow who was selling cigarettes illegally. Big protests broke out. The next day KMT soldiers fired on crowds.

Chiang's government denied that a massacre had occurred and stuck to this position long after Mao Zedong's forces drove the KMT from the Chinese mainland.

On the 30th anniversary of the massacre, a public reckoning with the event seemed no more likely than a full investigation into Tiananmen does today. But that changed.

Taiwan's past, China's future

Even though only one of the massacres can be openly discussed in the place in which it occurred, there are three ways in which thinking about 2/28's legacy helps put that of 6/4 into perspective.

First, by showing that the histories of authoritarian systems often take unexpected turns.

Hopes for a long-term relaxation of repression in the PRC have proved chimerical. The CCP has been remarkably resilient. But Taiwan's case reminds us that even resilient objects break.

The ROC was under martial law for decades. As late as 1979, the KMT used force to crush pro-democracy protesters in Kaohsiung, Taiwan's second biggest city. In the 1980s, however, democracy activists asserted themselves with a vigor that took many observers by surprise, and, in 1986, Chiang Ching-kuo, Chiang Kai-shek's son and successor, unexpectedly reversed his father's policies and allowed for formal establishment of an opposition party. Martial law eventually ended in 1987.

Second, multiple forces can drive change.

Political activists, long-time officials, and other actors played important roles in Taiwan's democratization. This suggests the need to avoid letting PRC President Xi Jinping's current emphasis on control engender myopia. Even when a confident strongman is on top, it is worth keeping an eye out for forces bubbling under the surface.

Third, change is a long slog.

It's worth remembering that the ROC did not have its first direct presidential election until 1996. And its first non-KMT President was not elected until 2000.

Taiwan's history does not tell us what will happen in the PRC in the near future, but it does give us reason to hope that policies -- including how 6/4 is remembered and discussed -- can eventually change.

The question that remains is how, and when?