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The criminal case against China's ex-security chief not only plays to public demands to curb corruption but spells the downfall of one of President Xi Jinping's biggest rivals, puts other challengers on their toes and leaves Xi more solidly in control than ever.

The fate of the once-feared Zhou Yongkang, 72, appeared to be sealed by the just-after-midnight announcements Saturday that he was expelled from China's ruling Communist Party and arrested in a criminal investigation into allegations ranging from bribe-taking to leaking state secrets.

"Many of Xi's enemies have been scared, and he's been successful in intimidating his enemies," said Willy Lam, an observer of China's elite politics at the Chinese University of Hong Kong. "All of them have become obedient — at least superficially — to Xi Jinping."

Zhou, with a face that looks like it is made of stone, was a former member of the party's all-powerful Politburo Standing Committee and was once in charge of the country's police, security forces and judiciary, a vast apparatus that spends more on domestic security than what China spent on the People's Liberation Army, the world's biggest military.

Zhou's status as security czar would have meant he had access to private phone conversations and secret information about national leaders. The state secrets allegations against him likely stem from his attempts to use leaks about colleagues to jockey for position ahead of China's handover of power in late 2012 to a new generation of leaders at the retirement of President Hu Jintao, Lam said.

If he ends up formally charged on that count, it also may give the court a reason to keep trial proceedings closed and thus limit any politically damaging public disclosures.

Lam said Zhou and his associates also may have been part of the biggest corruption ring since the Communist Party took over power in 1949. His vast network of protégés involved hundreds of officials and billions of dollars, Lam said.

Whispers that Zhou was in trouble began to circulate months before he was set to retire in November 2012. They grew louder when his favored protégé, a deputy party chief in Zhou's old stomping ground of Sichuan province, became the first major official to fall in the anti-corruption campaign that Xi has made hallmark of his tenure. That happened mere 18 days after the conclusion of the power-transfer ceremony.

One by one, Zhou's associates — in the oil industry, in police and in Sichuan province, where he had built strong power bases — were placed under investigation, making Saturday's announcement marking Zhou's fall all but expected.

"When the arrow leaves the bow, there is no turning around," Beijing-based independent scholar Zhang Lifan said.

Zhou has not been formally charged and the case has been referred by party investigators to criminal prosecutors. The investigators also cited his keeping mistresses as a reason for his expulsion from the party. Adultery is not technically illegal in China, but it is deemed by the Communist Party to be a serious violation of party rules because mistresses are considered to inevitably open a politician up to financial demands that lead to corruption.

Any trial would be expected to have a foregone conclusion with Zhou's conviction, because the outcomes of such high-profile trials are widely believed to be negotiated among top leaders ahead of time.

The prosecution of Zhou also will serve to "shock and awe" opponents to Xi's two-year anti-corruption drive, Zhang said.

"The campaign has moved many people's cheese, and it has been met with great resistance, especially from the mid- and lower-level officials," Zhang said. "It is entering into a logjam, and the announcement against Zhou can help break that stalemate."

Differing from the view that the prosecution of Zhou is primarily about factional rivalry, Li Cheng, director of the John L. Thorton China Center at Washington-based think-tank Brookings Institution, said the main objective is to tamp down pervasive corruption that has crippled public confidence in party rule.

"It's about the life and death for the party," Li said, calling the retired Zhou a "dead tiger" with little political relevance left. "The urgency is to change the public view of the Communist Party and to raise its prestige."

China's political system at the highest levels is opaque, but in recent decades has been seen to embrace a rule-by-consensus approach among topmost leaders that eschews the personality cult and turmoil of the Mao Zedong era.

Xi Jinping has reversed some of that trend with a higher-profile public persona and with his moves to swiftly consolidate power, including placing himself at the helm of panels that control the military, domestic security and reform efforts.

He is now seen by many observers to be the strongest leader since the time of Deng Xiaoping that lasted into the 1990s.

The fall of Zhou and his associates has helped score further points for Xi, portraying him as a heroic, resolute figure against corruption, said Perry Link, a sinologist at the University of California at Riverside.

Stata media have celebrated the prosecution of Zhou as strong evidence of the party's determination and its ability to root out corruption, and, as expected, members of the public have cheered.

"That's a success for Xi Jinping. It increases his popularity," Link said.

However, cynicism remains among many members of the public who believe that corruption extends beyond just Zhou and his circle.

Corruption charges, when used selectively, are the time-honored ploy to purge political opponents, Link said. "Why Zhou Yongkang, but not other members of the Standing Committee? That difference is political," he said.

Any suggestion that the crackdown can cure China's epidemic corruption is naive, because corruption cannot be effectively curbed without independent checks on the party's power, which China so far lacks, Link said.

"As long as you have the one-party rule, it won't change the basic problem."