The Communist Party of China has placed former public security chief Zhou Yongkang under investigation for suspected "serious disciplinary violation", a term that usually refers to corruption. But what does Zhou's fall mean?

Before his last post, Zhou had a long career in the oil industry. In 1996, he became the chief of petro-behemoth China National Petroleum Corporation and soon the Party secretary of Sichuan province. From 2002 to 2007, he was the minister of Public Security. In 2007, he became a member of the 17th Political Bureau Standing Committee of the CPC Central Committee. Thanks to his public mandate, Zhou is said to have overseen the expansion of the country's public security sector.

About a year ago, the central government launched an investigation into Zhou as part of a wider anti-corruption campaign following the trial of Bo Xilai, former Party secretary of Chongqing municipality, who was found guilty of corruption in September 2013.

Amid the leadership transition in 2012 and 2013, many Western "sinologists" claimed that the new leadership was too conservative to engage in anti-corruption reforms.

At the time, I argued that China was moving toward reforms and would take the anti-corruption campaign to a new level - focusing on the rule of law.

In March 2012, CPC General Secretary Xi Jinping spoke at the Central Party School about the importance of collective responsibility and the dangers of private interests. What the world has witnessed in China since then is a determined and broad anti-corruption campaign.

As Xi said in January 2013: "We must uphold the fighting of tigers and flies at the same time, resolutely investigating law-breaking cases of leading officials and also earnestly resolving the unhealthy tendencies and corruption problems which happen all around people."

China's anti-corruption campaign is not unique, but its massive scale is unprecedented in modern history.