In the seventh century, during the Arab siege of Constantinople, the Byzantines introduced "Greek fire." Consisting of a thick, flaming substance launched from cannons, it proved devastating during naval battles because the fire could not be extinguished and was said to burn Arab ships even below the water line.

In today's Pentagon parlance, that weapon would be known as a "disruptive threat"--something that comes out of left field to tilt the balance of power. Ryan Henry, the principal deputy undersecretary of defense for policy, says that this kind of capability "puts at risk military power as a key instrument of the broader U.S. national power."

Henry was a key architect of the Pentagon's Quadrennial Defense Review, completed in February. In it, defense officials analyzed the military and categorized threats into four groups: traditional, large militaries such as China's; "irregular" forces such as those fighting U.S. troops in Iraq; catastrophic weapons; and disruptive threats.

Past disruptive actions include the Soviet detonation of an atomic bomb in 1949 (earlier than expected) and the launch of Sputnik 1. But today, what could offer an adversary the shock-and-awe equivalent of Greek fire? Henry offers one scenario: the ability to "counter ballistic effects"--to shoot down U.S. missiles and other projectiles.

Beyond broad outlines, though, officials cannot offer much that is concrete. In its 2006 National Security Strategy report, the White House said that such threats could take the form of "biotechnology, cyber and space operations, or directed energy weapons" used in new ways. Other policy statements emphasize "unanticipated" challenges, noting the difficulties of planning for the unknown.

Some experts argue that the military authorities do not have a good, systematic way to recognize disruptive threats. The Defense Science Board, an influential group of outside advisers to the Pentagon, said in a March report that, although the military has undertaken "extensive activity in the area of disruptive challenges," it lacks "a comprehensive, coherent effort to identify and address these challenges." Instead the science board said that "much of the current effort addresses excursions to traditional challenges or approaches that the U.S. can use to disrupt adversary operations rather than the reverse."

The science board and others note that recent operations in Iraq and the larger war on terrorism should give the U.S. pause in trying to anticipate the unknown in the information age. According to an internal report commissioned last year by the Office of Force Transformation, a Pentagon think tank, today's battlefields "extend into cyberspace" and involve ubiquitous modern products such as cell phones, the Internet and garage-door openers. Such technology means that disruptive threats could be deployed quickly. As the analysis warns: "We no longer have the luxury of years of warning" as in a previous era.

Henry acknowledges that the quadrennial review showed that the Pentagon's bureaucracy was too rooted in the industrial age; its modernization, he believes, will enable the armed forces to better predict and prevent unexpected challenges. He also adds that U.S. intelligence agencies are "scanning the science and technology horizon" for future disruptive threats.

Perhaps the most effective answer is to push the technological envelope. Adversaries might then be too busy playing catch-up. According to the Office of Force Transformation report, "the new solutions to disruptive challenges lie less in trying to detect and catch them ... and more in making the U.S. military a more elusive target by increasing the rate at which it transforms" itself. As Ryan notes, during the cold war the U.S. enjoyed decades of dominance in one crucial area: stealthy aircraft, its own disruptive capability. "If the other side would have had it, and we didn't," he remarks, "we don't know what the outcome would have been, but we would have expended a significant amount of blood and treasure trying to defeat it."