Backup plans are, by definition, not the best-case scenario. But usually they can be counted on to be reasonably close to Plan A. Here's the cases where they were a couple million deaths off the mark:
  1. Operation Downfall!
    A proposed 1945 amphibious invasion of Japan, planned by members of the U.S. military whose security clearance was below the Manhattan Project’s level. It came so close to happening, troops were being moved into position toward the debarkation point on Okinawa when President Harry S Truman decided to drop the bomb. The Military was so certain things were about to go very badly for U.S. soldiers that 500,000 Purple Hearts were made in preparation.
  2. Operation Vulture!
    In the early 1950s, after years of bloody fighting to hold French Indochina, France asked for three American tactical nuclear weapons to turn that incipient communist region into New Hiroshima. Among the notable figures who signed off on it were U.S. secretary of state John Foster Dulles and Vice President Richard Nixon, who had actually helped draw it up.
  3. "Huele a Quemado"!
    Spanish for "It smells like something is burning", it was Panamanian General Omar Torrijos' plan to wreck the Panama Canal IF his 1977 negotiations for control of it didn't work out. United States President Jimmy Carter agreed to give up the Canal in the future year of 1999, preventing a surprise sabotage campaign by Panama's military and (experts predict) ensuing jungle war between Panamanian forces and a U.S. military coming off of the Vietnam quagmire.
  4. A joint Cold War moon landing!
    JFK wanted a joint US-USSR moon landing, in hopes of offloading some of the cost of it on Russia. He and Nikita Khrushchev had built up a trust, and Khrushchev saw what Kennedy saw (a chance to completely change the tenor of the space race and the Cold War at large) in the opportunity, so Nikita finally decided it was a good idea…on the exact week Kennedy was assassinated. Khrushchev didn’t trust Lyndon Johnson enough to keep the plan, and passed up a chance to wind down the Cold War early.
  5. Accidental nuclear armageddon!
    Follow this link to see how the world almost ended by mistake in 1983...and before that because of a clumsy bear...and anytime if a Soviet deathbot malfunctioned: