(...) This is the undisputed domain of General Keith Alexander, a man few even in Washington would likely recognize. Never before has anyone in America’s intelligence sphere come close to his degree of power, the number of people under his command, the expanse of his rule, the length of his reign, or the depth of his secrecy. A four-star Army general, his authority extends across three domains: He is director of the world’s largest intelligence service, the National Security Agency; chief of the Central Security Service; and commander of the US Cyber Command. As such, he has his own secret military, presiding over the Navy’s 10th Fleet, the 24th Air Force, and the Second Army.

Alexander runs the nation’s cyberwar efforts, an empire he has built over the past eight years by insisting that the US’s inherent vulnerability to digital attacks requires him to amass more and more authority over the data zipping around the globe. In his telling, the threat is so mind-bogglingly huge that the nation has little option but to eventually put the entire civilian Internet under his protection, requiring tweets and emails to pass through his filters, and putting the kill switch under the government’s forefinger. “What we see is an increasing level of activity on the networks,” he said at a recent security conference in Canada. “I am concerned that this is going to break a threshold where the private sector can no longer handle it and the government is going to have to step in.”

In its tightly controlled public relations, the NSA has focused attention on the threat of cyberattack against the US—the vulnerability of critical infrastructure like power plants and water systems, the susceptibility of the military’s command and control structure, the dependence of the economy on the Internet’s smooth functioning. Defense against these threats was the paramount mission trumpeted by NSA brass at congressional hearings and hashed over at security conferences.

But there is a flip side to this equation that is rarely mentioned: The military has for years been developing offensive capabilities, giving it the power not just to defend the US but to assail its foes. Using so-called cyber-kinetic attacks, Alexander and his forces now have the capability to physically destroy an adversary’s equipment and infrastructure, and potentially even to kill. Alexander—who declined to be interviewed for this article—has concluded that such cyberweapons are as crucial to 21st-century warfare as nuclear arms were in the 20th.

And he and his cyberwarriors have already launched their first attack. The cyberweapon that came to be known as Stuxnet was created and built by the NSA in partnership with the CIA and Israeli intelligence in the mid-2000s. The first known piece of malware designed to destroy physical equipment, Stuxnet was aimed at Iran’s nuclear facility in Natanz. By surreptitiously taking control of an industrial control link known as a Scada (Supervisory Control and Data Acquisition) system, the sophisticated worm was able to damage about a thousand centrifuges used to enrich nuclear material. (...)