World War III

World War III, also known as the Third World War, was a conflict that broke out on planet Earth. 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) Much of these tensions were caused by the Cold War. (TV: Cold War)

In the 1960s, the United States government built a moonbase on the far side of the Moon 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 British 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 London Underground radioactive. They knew this could trigger a nuclear war with the Soviet Union. 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 occurred. 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) The United States was also involved and the conflict started over Middle Eastern spheres of influence. (PROSE: Day of the Daleks)

In 1974, American President Richard Nixon intended to destroy Britain through nuclear war in order to prevent the Remoraxians from invading Earth.(COMIC: In With The Tide)

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 foiled 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 alternate timeline, WOTAN was successful in taking over the minds of much of the population in 1966. It was destroyed in an uprising in 1969 but left behind a broken world in which international communciation and cooperation became non-existent and numerous factions struggled for control. In response to the incident, Britain made radio communications illegal, which later proved to be a large disadvantage. World War III began in the following years, the main fight being between the United Kingdom and South Africa. South Africa made agreements with the Cybermen who occupied the Arctic in 1986, gaining weapons and technology which gave them a huge edge in the fighting. Outmatched, the British, led by the fascist General Louise Bamford, sought to win the war with time travel using Imperial Dalek artefacts recovered from the Shoreditch Incident. In 2006, the South Africans occupied France and began launching bombing raids against Britain as a precursor to an invasion, and when said invasion did come, the British were quickly overcome. Refusing to surrender, Bamford was killed by Griffiths, a double agent working for the South Africans. Their leader dead, the British hurridly began looking for someone to elect so that they may come to armistice terms with South Africa.

The First Doctor, with his companions Susan, Ian and Barbara, as well as Griffiths, averted this timeline when they used Britain's time travel hoop to travel back to October 1972. There, they both destroyed the Dalek artefacts before the time travel experiments could properly begin and had a more positive influence on a younger Bamford, convincing her to aid in the reconstruction of a more positive Britain and more positive world. (PROSE: The Time Travellers) Later in his personal timeline, the First Doctor defeated both WOTAN (TV: The War Machines) and the Cybermen (TV: The Tenth Planet) before either of them could contribute to a third world war.

In another 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 a third 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

 * The Time Travellers highly implies that the British and South African world war caused by WOTAN was 'supposed' to happen, taking place an original timeline of Earth's history. The First Doctor averting this war and then later defeating WOTAN changed history, creating an 'alternate' timeline in which this third world war never happened, and this became the prime timeline for the Doctor Who universe. The proper Third World War therefore became the pre-25th century war alluded to in Frostfire.
 * In 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'."