First appearance of the often used "fail-safe" control box.
President Talbot (Ford Rainey) orders that the (supposed) incoming missiles be destroyed "if they cross the D.E.W. line". The D.E.W - or Distant Early Warning line - was a chain of radar stations across the U.S. and Canadian Arctic, operating from the late 1950s through the early 1990s, watching for any possible incursion from the Soviet Union or China.
This episode takes place in the 1970s.