After the disastrous Battle of Sedan during the Franco-Prussian War of 1870, French military planners had to recover from that catastrophic loss and the humiliating political aftermath in order to rebuild the army into one of the first-rate armies of Europe. In contrast to the German penchant for detailed operational plans of battle, French military officers began to espouse a doctrine of flexible offensives.
The army, it was thought, would retain its momentum through constant attack of the enemy lines, and ruthlessly exploiting any weakness that followed. The enemy would never be able to catch their balance, initiative would stay on the part of the French, and the grinding advances of the German enemies would be checked by lightning counter-thrusts. This would apply from the battalion level up to the theater level – where the invader was weak, there the French would attack.
It is one of the great paradoxes of August 1914 that even as the French staggered along the front in the Low Countries, they were meeting with advances through Alsace and Lorraine, but these had to be checked to provide reinforcements to save Paris. By the end of that year, the spirit of “always attack” had faded, and the trenches defined the Western Front.