When Britain returned Hong Kong to Chinese sovereignty, in a 1997 handover agreement that ended an era of British colonialism dating to the Opium Wars, China’s top leader at the time, Deng Xiaoping, guaranteed Hong Kong’s way of life for at least 50 years.

Deng was the chief architect of the policy for Hong Kong known as “one country, two systems.”

The policy was the blueprint for preserving the prosperity and autonomy of a freewheeling capitalist enclave on the doorstep of the Chinese Communist mainland which had become critical for financial and trade links to China’s own ambitious economic future. The policy also bolstered China’s image as an increasingly responsible force in the world.

Within a matter of years, however, China began taking steps that eroded the liberties enjoyed by Hong Kong’s 7.5 million people, and the moves have increasingly led to pro-democracy demonstrations and suspicion toward the intent of the Chinese Communist authorities in Beijing.

On Thursday, those authorities announced the most sweeping step yet, with proposed security laws that could effectively subvert Hong Kong’s remaining freedoms and bring it under full Chinese control.