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The laws of cyberwar

Inside the new effort to set rules for a battlefield no one can see

Amid the flurry of national security documents leaked in the last weeks was one that got less attention than the PRISM surveillance program, but also opened a window on a secretive new realm: Presidential Policy Directive 20, issued last October, which instructed national security and intelligence officials to develop America’s capacity to wage cyberwarfare.

Cyberwarfare is emerging as the latest uncertain frontier of international relations, a way for developed nations to attack one another without appearing to do anything at all. The issue lay at the center of President Obama’s recent meeting in California with China’s leader, Xi Jinping: Obama confronted Xi about cyberattacks on government and corporate websites apparently hacked by a secret directorate of the Chinese military, People’s Liberation Army Unit 61398. The United States, of course, has its own alleged history of cyberattacks: It is believed to have helped launch the Stuxnet computer worm against Iran’s nuclear program several years ago.

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