Whether to do with Tibet, Taiwan, or Falun Gong, a spiritual sect, there are plenty of these, leading to sites deemed "harmful" by China. In a year of harder economic times and sensitive political anniversaries, the authorities are especially edgy. The thin and cautious reporting in the press of events in Iran suggests they are also nervous about access to news of political protest elsewhere.

Despite the opposition, however, which includes a grassroots attempt to organise an internet boycott in China on July 1st, the government remains undaunted, promising that technical flaws will be fixed and that Green Dam will go forward. It has also opened a second front, lashing out at Google for including a feature in its Chinese service that automatically completes search-query terms - it complains that this can lead users to sites containing pornography.

Google has long struggled to reconcile its corporate credo ("Don't be evil") with the onerous demands of China's internet regulators. It has promised to renew its efforts to keep in line with Chinese standards. But the company also has fair cause to wonder why it has been singled out. Its main Chinese competitor, Baidu, is just as good at finding smut.

© The Economist Newspaper Limited 2009