Set after the events of Avengers: Age of Ultron, Captain America: Civil War shows a world in which the civilian population has suffered catastrophe after catastrophe. Alien invasions, plummeting cities, general super-powered destruction and death.
In order to deal with growing public outrage at collateral damage, the UN takes control of the Avengers. Those who sign up can only intervene in situations when instructed. Those who refuse to sign are arrested, or forced to retire.
Captain America (and friends) end up on the other side from Iron Man (and different friends). While they fight over philosophical differences, future Nazis search for cryogenically-frozen super-soldiers.