The "front" was where the battle/fighting takes place. Everything happens simultaneously. Troops & "materiel" are brought in from rear securely held positions. In modern warfare bombing would be taking place anytime along the front lines or behind the enemy held lines in enemy held territory. As far as major battles taking place as in ww2, the forces would gather at the front lines & a build-up of men & materiel takes place until one or the other side makes the attack, at some crucial/key location necessary to defend territory or advance towards an objective. Usually the more prepared opponent. One side will be defending a position. Napoleon said the side which remained within its defenses would likely loose the battle. This battle would last until one side has its center or "center of gravity" knocked out from under & is no longer capable of fighting as a co-ordinated cohesive entity. Until this occurs, troops, reinforcements, materiel are all being fed into the battle, there being a long line of communication extending back to source of the supply lines originate. Dozens of support personnel are required for each frontline combat soldier. Of course, in a "dramatic" portrayal, this mundane aspect will never be touched upon. Hours of boredom interspersed with moments of sheer terror at the front. None of this movement buildup in support of/preparing for an attack or possible enemy attack at the front lines ever stops. If one side cannot keep up momentum, in the long run, prospects don't look good for them. Ability to produce & bring massive amounts of ground forces, planes, tanks, ships & supplies, in "conventional" warfare is crucial. Notwithstanding, other types of warfare are taking place at the present which are not as obvious tho they are bringing about the desired results.