When does war escalate?

War is said to escalate when it increases rapidly in scale or intensity. the word was added to the military vocabulary by the Americans during the Vietnam War to denote the increasing degrees of United States involvement in the fighting. It has also been used by military writers to describe the development of a possible war between two powers through successive stages from the use of conventional weapons to the localized employment of atomic weapons and finally, in all-out exchange of annihilating nuclear missiles.