Military deception (MILDEC) is an attempt by a military unit to gain an advantage during warfare by misleading adversary decision makers into taking action or inaction that creates favorable conditions for the deceiving force. This is usually achieved by creating or amplifying an artificial fog of war via psychological operations, information warfare, visual deception, or other methods. As a form of disinformation, it overlaps with psychological warfare. Military deception is also closely connected to operations security (OPSEC) in that OPSEC attempts to conceal from the adversary critical information about an organization's capabilities, activities, limitations, and intentions, or provide a plausible alternate explanation for the details the adversary can observe, while deception reveals false information in an effort to mislead the adversary.

Deception in warfare dates back to early history. The Art of War, an ancient Chinese military treatise, emphasizes the importance of deception as a way for outnumbered forces to defeat larger adversaries. Examples of deception in warfare can be found in ancient Egypt, Greece, and Rome, the Medieval Age, the Renaissance, and the European Colonial Era. Deception was employed during World War I and came into even greater prominence during World War II. In modern times, the militaries of several nations have evolved deception tactics, techniques and procedures into fully fledged doctrine.

Many standard military activities can be considered deceptive, but not deception. For example, a unit may move into an assembly area to complete organizing and rehearsing prior to a mission. It is a standard deceptive tactic to camouflage the vehicles, equipment and personnel in the assembly area with the intent of confusing the enemy. Military deception is more complex than simple deceptive activities, with a unit deliberately planning and carrying out an elaborate effort that will cause a targeted adversary decision maker to take an action that is detrimental to the adversary and beneficial to the side employing deception. The exception in US doctrine is Deception In Support of OPSEC (DISO); rather than cause an adversary decision maker to take a specific action, DISO augments a friendly unit's Operations Security (OPSEC) plan to aid in concealing the friendly unit's most important mission details.

In US doctrine, the three categories of deception are Joint Military Deception (MILDEC), Tactical Deception (TAC-D), and Deception In Support of OPSEC (DISO). In TAC-D, tactical-level commanders attempt to cause an enemy decision maker to act in a way that is unfavorable to the enemy and beneficial to the friendly tactical commander's objectives. Joint MILDEC is conducted to support military campaigns and major operations at the strategic level. It causes adversaries to take actions or inactions that are favorable to the friendly commander's objectives, typically at a unified combatant command level.

With DISO, the side employing deception enhances its OPSEC by causing the adversary's information and intelligence gathering and processing capability to either not notice important details of the deceiving force's operations, or to misinterpret those details. The benefit to the deceiving side is gained by preventing the adversary from gaining timely and accurate situational awareness.

Deception can be accomplished through either increasing or decreasing an adversary's understanding of the operating environment. Ambiguity increasing deception is intended to sow confusion in the mind of the enemy decision maker by presenting multiple possible friendly courses of action. Because the adversary does not know which is true, his reactions are delayed or paralyzed, which gives the friendly side an advantage. With ambiguity decreasing deception, the friendly side intends to make the adversary certain of the friendly course of action — certain, but wrong. As a result, the adversary will misallocate time, personnel, or resources, which enables the friendly side to obtain an advantage.

The Operation Bodyguard deception in World War II can be viewed as an ambiguity increasing deception that over time became ambiguity decreasing. Initially, the aim was to increase confusion among German planners and leaders by presenting the possibilities of Allied invasions at the Pas-de-Calais and Normandy in France, as well as the Balkans, southern France, and Norway. Eventually, the deception increased certainty on the German side by causing them to conclude that Calais was the real invasion site. When the Allies attacked at Normandy, they did so with the advantage of surprise.

Military deception may take place at the tactical, operational, and strategic levels of warfare. The five basic tactics include: