The U.S. has publicly and privately warned China that America would begin fighting back in response to continuing Chinese efforts, via the Internet hacking to spy, steal and suppress media in the West, especially the United States. This became more of an issue after China recently began using state sponsored Internet censorship technology to shut down web sites outside China that Chinese officials found “offensive”. Attacks against anti-Chinese sites has been going on for years but the latest effort, using massive DDOS attacks on foreign websites, gave the Chinese effort a name; the “Great Cannon”.
This was seen in the West as an act of war. This, the U.S. pointed, was crossing a line and retaliation was now a possibility. The Chinese were caught using their own resources (the Great Firewall of China and other Chinese Internet companies) to shut down foreign websites with DDOS attacks. Ironically the techniques used to deploy the Great Cannon were first made public when an NSA employee released a large number of NSA documents, one of which described how something like the “Great Cannon” would work. While the U.S. never used those techniques, China has and seems willing to keep escalating its use of the Internet to get whatever it wants. China appears to feel that the West is unwilling (but certainly, as the leaked NSA documents showed, not unable) to reply.
This sort of Chinese misbehavior was no secret in the West. It was known, for example, that in 2011 a Chinese state run TV station removed a video from its web site that showed (apparently by accident) a Cyber War tool (that can launch a DDOS attack on another site and shut it down temporarily.) The government denied that the Cyber War program was government property, but refused to comment further. The video first appeared on TV a month before the “error” was discovered and the video removed. China still officially denies this misbehavior but it is no secret to most Chinese, who are rather proud of China sticking it to the West with a weapon (the Internet) invented in the West. As more in the West become aware of this there has not been a great increase in popular support for actual retribution. So American threats of retaliation do not carry much weight with the Chinese leaders who are selecting new targets for the Great Cannon.