HONG KONG--A year ago, an extradition bill in Hong Kong that could have sent suspects to mainland China for trial sparked the largest protests and biggest political crisis the semiautonomous territory has seen since its return to China in 1997.

Now, the issue has come full circle. After China used the demonstrations as justification to impose a sweeping new security law in Hong Kong, the United Kingdom this week became the fourth country to suspend its extradition treaty with the former British colony, joining the United States, Australia and Canada.

As with Hong Kong’s now-withdrawn extradition bill, the concern was the possibility that people could be handed over to mainland Chinese law enforcement and disappear into its opaque and frequently abusive legal system.

“Extradition, at the bottom of it, is a political act. It’s a political act whether or not you surrender a person,” said Philip Dykes, chairman of the Hong Kong Bar Association. “Extradition treaties with Hong Kong were always on the basis that whatever happens, a person will not be removed to the mainland.”

The moves underscore a growing divide between authoritarian China and the United States and other like-minded democracies over human rights and other issues. Just three years ago, Australia’s conservative government was making a high-profile push for an extradition treaty with China, an effort that ran afoul of parliamentary opposition. Now, not only has Australia suspended extradition with Hong Kong, it is warning its citizens of the risk of arbitrary detention in China.

U.S. Secretary of State Mike Pompeo, after meeting U.K. Prime Minister Boris Johnson and Foreign Secretary Dominic Raab in London, applauded Britain’s suspension of the extradition treaty and other recent steps against China.

“We want every nation to work together to push back against the Chinese Communist Party’s efforts in every dimension that I have described,” he said.

China says the new security law is needed to combat terrorism and separatism and prevent Hong Kong from becoming a base for undermining Chinese state power. In general, cases would be tried in Hong Kong, but the law allows for mainland jurisdiction in some circumstances.

The legislation that launched last year’s protests was triggered by a murder case. A young Hong Kong man, Chan Tong-kai, allegedly killed his girlfriend while on vacation in Taiwan before fleeing home.

Hong Kong authorities could not send Chan to Taiwan for trial without an extradition agreement. The government cited Chan’s case as an example of the loophole the extradition legislation would close, allowing Hong Kong to transfer fugitives to any jurisdiction with which it did not have a treaty, including Taiwan and mainland China.

The proposal triggered a massive backlash from Hong Kong residents, who feared that suspects could be sent to the mainland for trial.

While the Hong Kong legal system’s fairness and transparency has helped establish the city as a center for business and finance, China’s Communist Party-dominated courts are accused of handing down convictions based on political considerations and using coerced confessions.

Though the government eventually withdrew the bill, the demonstrations took on a broader anti-government and anti-police agenda and grew increasingly violent.

China’s detention of several Hong Kong booksellers in late 2015 had already focused concern on the undermining of the legal autonomy the territory was promised when the former British colony was handed back to China in 1997.

The booksellers vanished before resurfacing in police custody in mainland China. Among them, Swedish citizen Gui Minhai was abducted from his vacation home in Thailand and later appeared twice in videotaped confessions, the second time after being taken off a train by police in eastern China in January 2018 while in the company of two Swedish diplomats.

Another of the booksellers, Lam Wing-kee, later fled to Taiwan.

The prospect of falling afoul of the new law and disappearing into the Chinese legal system is prompting others to leave as well. Nathan Law, a leading member of Hong Kong’s opposition movement, fled to Britain after the law was enacted.

Simon Cheng, a former employee of the British Consulate in Hong Kong who had left earlier, was granted political asylum in Britain earlier this month.

China has pledged to retaliate for Britain’s decisions to cancel its extradition treaty with Hong Kong as well as ban the sale of military-grade equipment to the territory.

“China urges the British side to abandon the illusion of continuing colonial influence in Hong Kong ... so as to avoid further damage to China-Britain relations,” foreign ministry spokesperson Wang Wenbin said Tuesday.

The U.K.-Hong Kong extradition agreement has not been widely used. In the last decade, no one was extradited from Britain to Hong Kong, and only three people were sent in the other direction, according to the Hong Kong government.

China’s dissatisfaction is based on issues of national pride as well as more practical concerns. Under President Xi Jinping, China has pushed hard for the return of corrupt officials and others who have fled abroad with their ill-gotten gains. While that effort has scored some successes, it has been frustrated by the lack of extradition treaties with key countries.

Canceling extradition to Hong Kong represents a further vote of no-confidence in China’s legal system, one already registered by the refusal of the United States, Britain and other nations to sign extradition agreements with Beijing.

Australia abandoned plans to do so in 2017. Parliament’s endorsement of the treaty was to be a highlight of Chinese Premier Li Keqiang’s visit to Australia in March of that year, a high point in a volatile diplomatic relationship. Since then, ties have deteriorated to a historical low.