War

A war was a type of conflict.

According to Major Brogan, "War does terrible things to good people", by bringing out extremes in emotion and behaviour. (AUDIO: Innocence) The Fifth Doctor believed that "war can make monsters of the best of us". (AUDIO: Empire of the Racnoss)

Though the goal of a war was victory over the other side, (TV: The Armageddon Factor, AUDIO: Purity) the Thirteenth Doctor held that "No one ever wins at war." (PROSE: The Good Doctor)

War was an expensive venture, (TV: The Armageddon Factor) and it had its victims. (AUDIO: Purity)

The Twelfth Doctor stated wars, after massive casualties and destruction, always ended with people doing what they should have done from the beginning: "Sit down and talk!" (TV: The Zygon Inversion)

"When you fire that first shot, no matter how right you feel, you have no idea who's going to die! You don't know whose children are going to scream and burn. How many hearts will be broken! How many lives shattered! How much blood will spill until everybody does until what they were always going to have to do from the very beginning: Sit down and talk!"

- Twelfth Doctor

Article 15 of the Shadow Proclamation said that murder was not a rule of war. (PROSE: Diamond Dogs)

Conflicts
The Time Lords warred with the Great Vampires during the Dark Times. (TV: State of Decay) They also engaged in two major time wars which appeared to destroy Gallifrey: the War in Heaven against the enemy, (PROSE: Alien Bodies, The Ancestor Cell) and the Last Great Time War against the Daleks. (TV: Dalek, The Day of the Doctor)

Earth had at least six world wars: Two in the 20th century, (TV: Twice Upon a Time, PROSE: Timewyrm: Exodus) two between the 20th and 25th centuries, (AUDIO: Frostfire) one in the 37th century (COMIC: Art Attack) or the 49th century (AUDIO: Singularity) and one in the year 5000 (TV: The Talons of Weng-Chiang; AUDIO: The Butcher of Brisbane) The Ninth Doctor mentioned he was present at World War V. (TV: The Unquiet Dead)

After the First Doctor helped defeat the Daleks during the First Dalek War, his granddaughter Susan stayed on Earth with resistance fighter David Campbell. (TV: The Dalek Invasion of Earth, Prelude Deceit)

The War Lords kidnapped humans from various wars in Earth's history to create an army to conquer the galaxy. (TV: The War Games)

The Rutans and Sontarans were at war with each other for thousands of years. (TV: The Time Warrior, The Sontaran Experiment, TV: Horror of Fang Rock, The Poison Sky)

Just prior to invasion of Earth by the New Dalek Empire in 2009, General Sanchez, director of UNIT's base in New York City, announced "ladies and gentlemen, we are at war!" (TV: The Stolen Earth) Similarily, in the face of an attack on the Game Station by the Daleks in the year 200,100, Jack Harkness announced "this is it, ladies and gentlemen, we're at war!" (TV: The Parting of the Ways)