FIVE MONTHS after anti-government demonstrations first erupted over an extradition law, Hong Kong is on the brink of breakdown. All this week, police fought pitched battles with students who had barricaded university campuses and armed themselves with bricks, firebombs and even bows and arrows. When the students finally evacuated the campuses Saturday, Chinese troops left their barracks to dismantle barricades — the first time they had taken to Hong Kong’s streets during the crisis. Hospitals have treated scores of people for injuries, many of them serious.

All this is happening in a city once known for law and order, and for the peaceful moderation of its pro-democracy movement. The reason is unquestionable: the implacable hard line taken by the Chinese Communist government and the Hong Kong administration it controls toward what began as peaceful protests.

The demonstrators who first turned out on Hong Kong’s streets in June had a simple demand: the withdrawal of a proposed law that would have allowed Hong Kong authorities to arrest and turn over people to mainland authorities. The unrest could have ended quickly had the law been dropped; instead, Hong Kong chief executive Carrie Lam refused for months to concede, even as tensions steadily escalated.

By now, having been assaulted every weekend by police with tear gas, rubber bullets, water cannon and, most recently, live ammunition, protesters have escalated their demands. They want an independent investigation of police brutality and the dropping of charges against the thousands who have been arrested. Most important, they want Beijing to deliver on the promise it made at the time it took control of Hong Kong in 1997 and allow fully democratic elections for its executive and legislature.

Once again, the regime has taken an uncompromising stand, rejecting any concession. And so the demonstrations have spread from weekends to weekdays, and protesters are increasingly defending themselves from the police with their own violence. That’s regrettable. But the students did not initiate the fighting, nor can they end it. The responsibility for de-escalation lies with Beijing. The only way to end the street fighting is for Hong Kong authorities to restrain the police and meet the demonstrators’ reasonable demands.

Unfortunately, the regime probably will resort to more drastic measures of force. If so, the United States and other democracies must be ready to respond. The Hong Kong Human Rights and Democracy Act, which would require an investigation of repression in the city and sanctions against those responsible, is pending in the Senate and could be approved Monday if no senator objects. None should.

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