Americans’ broad dependence on computers first became clear in the late 1990s, when reports of the Y2K problem spawned predictions of nationwide chaos and societal breakdown. In the decade since, that dependence has only deepened. Cyberspace—now used as a catchall term to refer to the entire domain of networked computers and electronic devices—is used every day for functions vital to the U.S. economy, society, government and military. And those who would threaten U.S. national security have noticed. The Center for Strategic and International Studies’ Commission on Cybersecurity for the 44th Presidency stated as its central finding that the United States “must treat cybersecurity as one of the most important national security challenges it faces. . . . This is an issue on par with weapons of mass destruction and global jihad.”Although the publicly available details are few, many sources report a startling number and variety of probes, attacks and data thefts by hackers, criminals and spies on the computers and networks of government agencies, large corporations and many others.4 These everyday occurrences are just the beginning of the challenge, however. In April 2007, the Baltic nation of Estonia suffered a large, sustained campaign of cyber attacks that crippled the networks of its government, banks, news outlets and other organizations. Although the subsequent investigations were largely inconclusive, the attacks were widely believed to have been instigated by the Russian government. The Caucasian nation of Georgia suffered a similar cyber warfare campaign during its short conflict with Russia in August 2008. Servers in South Korea and the United States sustained a series of attacks in early July 2009 that some blamed on North Korea. Cyber warfare has been part of the Chinese military’s strategic thinking since 1997. Indeed, the Chinese are suspected of aggressively probing U.S. networks in recent years, attempting to “scout the terrain,” gather information and lay the groundwork for any future conflict. Many other countries are also developing cyber warfare capabilities, and non-state actors surely are as well. Tomorrow’s wars will be waged in cyberspace as well as in real space—and perhaps in cyberspace alone.Some defense experts discuss cybersecurity within the conceptual framework of the “global commons”: “those areas of the world beyond the control of any one state—sea, space, air and cyberspace—that constitute  the fabric or connective tissue of the international system.”U.S. grand strategy has long recognized the benefits of keeping such commons safe, secure and open to all. Maintaining a free and open Internet—an international resource akin to the oceans—has become vital to U.S. interests. Like the oceans, cyberspace must be policed by all, but the United States, as a major beneficiary of all that cyberspace has to offer, should take the lead—vigorously and without delay.