One of these deficiencies involved the Minuteman silos, whose internet connections could have allowed hackers to cause the missiles’ flight guidance systems to shut down, putting them out of commission and requiring days or weeks to repair.

These were not the first cases of cybervulnerability. In the mid-1990s, the Pentagon uncovered an astonishing firewall breach that could have allowed outside hackers to gain control over the key naval radio transmitter in Maine used to send launching orders to ballistic missile submarines patrolling the Atlantic. So alarming was this discovery, which I learned about from interviews with military officials, that the Navy radically redesigned procedures so that submarine crews would never accept a launching order that came out of the blue unless it could be verified through a second source.

Cyberwarfare raises a host of other fears. Could a foreign agent launch another country’s missiles against a third country? We don’t know. Could a launch be set off by false early warning data that had been corrupted by hackers? This is an especially grave concern because the president has only three to six minutes to decide how to respond to an apparent nuclear attack.

This is the stuff of nightmares, and there will always be some doubt about our vulnerability.