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)
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!
The Third Doctor warned a Thal expedition working against the Daleks not to glorify their stories upon their return to Skaro, to avoid the prospect of war sound exciting to the largely peaceful race. (TV: Planet of the Daleks) He also acknowledged, however, "the sad truth" that war often bred progress, and that without any such dynamic, many species and civilisations risked remaining static. (PROSE: Catastrophea)
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)
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)