CAMBODIA'S NORMALLY repressive leader, Hun Sen, is on a tear even by his standards. In recent weeks he has outdone himself in destroying what remains of independent news media, civil society and political opposition. His apparent motive is to wipe out any contrary voices before a July 2018 election, transforming a malfunctioning democracy into a fully authoritarian state.

The latest turn of the screw came Sunday, with the arrest of opposition leader Kem Sokha and the announcement two days later that he had been charged with treason for "a secret plan and the activities of conspiracy." His "red-handed crime" was his appearance in a 2013 video telling supporters he received U.S. support and advice in planning political strategy. His lawyer, Pheng Heng, denied this was a crime, telling Reuters: "What he talked about was elections in a multi-party democratic way." Kem Sokha's arrest is a severe setback for his Cambodia National Rescue Party, previously headed by Sam Rainsy, who resigned earlier this year and is in exile. The party, established in 2012, posed the first real challenge in years to Hun Sen's rule. No wonder it is now in his crosshairs.

Another target is the Cambodia Daily, a newspaper known for its critical investigative reporting and fierce independence. Faced with a one-month deadline from the government to pay $6.3 million for years of back taxes, which the paper disputed, the Daily closed its doors Monday with a huge front-page headline in the last edition: "Descent Into Outright Dictatorship." The paper, founded by American journalist Bernard Krisher in 1993, was already in a perilous financial condition, and officials said the tax threat effectively forced its closure. At the same time, the government has been actively attempting to silence radio broadcasts, forcing dozens off the air; it is also pressuring radio station owners to stop relaying broadcasts of Radio Free Asia and Voice of America.

On Aug. 23, Cambodia ordered the National Democratic Institute, a nongovernmental organization loosely affiliated with the Democratic Party in the United States, to cease operations and its foreign workers to leave the country. The NDI had worked in Cambodia for 25 years, with both the ruling party and opposition, attempting to help strengthen democratic processes and institutions. Its expulsion was based on a 2015 law on associations and nongovernmental organizations that has been widely criticized as designed to serve as a weapon against such groups.

Cambodia's real patron is authoritarian China, which has been generous with aid and praised the Hun Sen regime this week for its efforts to "uphold national security and stability." This is Chinese code for imprisoning critics. The State Department expressed "grave concern" about the arrest of Kem Sokha, but at the same time President Trump has declared that the United States will not try to build democracy in other nations. "We are not asking others to change their way of life," Mr. Trump said recently. Hun Sen must think now is a good time to shutter what's left of Cambodia's democracy.