World War III

World War III, also known as the Third World War, was a conflict that broke out on planet Earth. The details are unknown. Both it and World War IV occurred prior to the 25th century. (AUDIO: Frostfire)

Preparations
During the second half of the 20th century and into the 21st century, fear of a third world war was usually connected to it being a potentially civilisation-destroying nuclear war. (AUDIO: Protect and Survive)

In the 1960s, the United States government built a moonbase on the far side of the Moon in the 1960s using alien technology. It was named after U.S. President Dwight D. Eisenhower. The moonbase was designed to contain a viable minimum population. In the event of a nuclear war, the plan was for its inhabitants to eventually return to Earth and rebuild human civilisation. It was destroyed by the Sixth Doctor on 23 November 1963. (AUDIO: 1963: The Space Race)

The United Kingdom kept open the four World War II "citadel" bunkers in London, so the British government could continue operating in a nuclear war. (The Secret Service quietly built a fifth) Sir Toby Kinsella darkly noted that the government wouldn't be interested in fighting aliens after World War III, instead wanting all the help it could get. (AUDIO: The Fifth Citadel)

Thwarted wars
Citadel Five threatened that unless Britain disarmed its nuclear arsenal, they would flood their reactor and leave the Underground radioactive. They knew this could trigger a nuclear war with the Soviets. This was prevented through deception by Sir Toby. (AUDIO: The Fifth Citadel)

The Doctor prevented this war from breaking out prematurely on several occasions in the late 20th century, (TV: The Mind of Evil, Robot, PROSE: Revolution Man) in two occasions ensuring that an alternate timeline where the war did happen never occured. Due to the actions of the psychic "Revolution Man" from 1967 to 1969, the nuclear powers grew tenser until missiles were actually fired on 18th May 1969; the Eighth Doctor stopped the missiles from impacting. (PROSE: Revolution Man) Later, though earlier for the Doctor, war almost broke out between China and the Soviet Union; fighting broke out in southern Asia and South America before Sir Reginald Styles was able to negotiate peace and the crisis cooled. The Third Doctor prevented a group of time-travellers from accidentally assassinating Styles and causing the war. (TV: Day of the Daleks) (In the novelisation, the US was involved as well and the conflict started over Middle Eastern spheres of influence) An early Third World War

In 1974 President Richard Nixon intended to destroy Britain through nuclear war in order to prevent the Remoraxians from invading Earth.(COMIC: Prisoners of Time)

Adam Mitchell claimed that he nearly caused World War III in the late 1990s, when he hacked into American Department of Defense computers. (TV: Dalek)

The Slitheen family tried to cause World War III in 2006 so they could sell Earth's radioactive remains as fuel. They nearly succeeded but were killed when Mickey Smith blew up 10 Downing Street with a missile on the Ninth Doctor's orders, killing them. (TV: World War Three)

In 2084, the Silurians attempted to start a war between the contemporary (human) superpower blocs. The Fifth Doctor stopped them firing Sea Base 4's nuclear missiles. (TV: Warriors of the Deep)

Alternative timelines
In an alternative timeline created by the Elder Gods in the hope of destroying Earth, Vladimir Kryuchkov became the General Secretary of the Communist Party of the Soviet Union instead of Mikhail Gorbachev in 1985. On 9 November 1989, World War III broke out between the United States and its allies including the United Kingdom on the one hand and the Soviet Union on the other hand. Given that nuclear weapons were used by both sides, hundreds of millions of people were killed in the conflict. This timeline was ultimately negated by the Seventh Doctor except for in a pocket universe where the Doctor trapped the two Elder Gods responsible and, accidentally, his companions Ace and Hex. (AUDIO: Protect and Survive)

In another alternative timeline, it was thought that U.S. President John F. Kennedy would have started a nuclear war on 22 January 1964 if he had not been assassinated on 22 November 1963. (PROSE: Who Killed Kennedy)

In another timeline, a 22nd century anti-Dalek resistance fighter named Shura travelled back to the 1970s and set off a dalekanium bomb which destroyed the World Peace Conference. World War III started as various global factions accused each other of having done so. In the aftermath, the Daleks conquered and ruled over the survivors, resulting in the world that Shura had tried to prevent from coming to pass in the first place. However, the Third Doctor undid the damage by ensuring that when the bomb went off, the delegates had already left. The Daleks and Ogrons who had travelled back to the conference were killed by Shura's bomb. (TV: Day of the Daleks)

Behind the scenes

 * In REF: The Doctor: His Lives and Times, it's strongly implied that Warriors of the Deep did indeed cause the Third World War: a Breaking News tweet reports, "Minister for Peace says world war is now 'inevitable' following discovery of wrecked Sea Base 'full of corpses'."