Answer: WarGames

Today the concept of “DEFCON” (or Defense Readiness Condition) is firmly enmeshed in the public consciousness to the point that modern movie viewers simply expect any movie with a heavy military presence or theme and a suitable stressor to result in at least someone yelling about what DEFCON level the United States is currently at.

Although the DEFCON system was put in place in 1959, the system was almost entirely unknown to the public until it was featured prominently in the 1983 film WarGames. The film follows the adventures of David Lightman (played by Matthew Broderick), a young computer hacker who accesses a U.S. military supercomputer designed to predict the possible outcomes of nuclear war. Lightman runs a simulation on the computer only to discover that it is not, in fact, a simulation but that the supercomputer actually controls the U.S. nuclear arsenal and as a result of his actions, almost causes World War III.

Throughout the film the DEFCON ranking is referenced frequently and that, combined with the wide popularity of the movie, was enough to inject the concept of DEFCON into the public imagination.

For the curious, there are five DEFCON levels ranging from 1 through 5. Despite how infrequently it is misused as indicating the most-dangerous level, DEFCON 5 is the least severe state and indicates the lowest (or normal) state of readiness. The military just performing daily standard operations is at DEFCON 5. Each level above DEFCON 5 indicates an increased level of readiness until you reach DEFCON 1, which is the maximum state of military readiness and also the descriptor used for imminent nuclear war.

Since the implementation of the DEFCON system, the highest confirmed DEFCON level the United States military has entered into is DEFCON 2, which it did during the 1962 Cuban Missile Crisis when the Strategic Air Command was set to DEFCON 2 for nearly a month.