Economy of force is the principle of employing all available combat power in the most effective way possible, in an attempt to allocate a minimum of essential combat power to any secondary efforts. It is the judicious employment and distribution of forces towards the primary objective of any person's conflict.

No part of a force should ever be left without purpose. The allocation of available combat power to such tasks, like limited attacks, defense, delays, deception or even retrograde operations is measured, in order to achieve mass at decisive points elsewhere on the battlefield.

It is one of the nine Principles of War

Principles of War

The Principles of War were tenets originally proposed by Carl von Clausewitz in his essay Principles of War, and later enlarged in his book, On War. Additionally, Napoléon Bonaparte had pioneered the "Principles of War," and "The armies of today are based on the organization created by Napoleon...

, based upon Carl von Clausewitz

Carl von Clausewitz

Carl Philipp Gottfried von Clausewitz was a Prussian soldier and German military theorist who stressed the moral and political aspects of war...

's approach to warfare. He once said that "Every unnecessary expenditure of time, every unnecessary detour, is a waste of power, and therefore contrary to the principles of strategy."

The Principles of War are a part of US Army doctrine. The current doctrinal manual for Army Operations is FM (Field Manual) 3-0, which defines, and describes, Economy of Force as follows:
Allocate minimum essential combat power to secondary efforts. Economy of force is the reciprocal of mass. It requires accepting prudent risk in selected areas to achieve superiority — overwhelming effects — in
the decisive operation. Economy of force involves the discriminating employment and distribution of forces. Commanders never leave any element without a purpose. When the time comes to execute, all elements should have
tasks to perform.