A military exercise, training exercise, or war game is the employment of military resources in training for military operations. Military exercises are conducted to explore the effects of warfare or test tactics and strategies without actual combat. They also ensure the combat readiness of garrisoned or deployable forces prior to deployment from a home base.

While both war games and military exercises aim to simulate real conditions and scenarios for the purpose of preparing and analyzing those scenarios, the distinction between a war game and a military exercise is determined, primarily, by the involvement of actual military forces within the simulation, or lack thereof. Military exercises focus on the simulation of real, full-scale military operations in controlled hostile conditions in attempts to reproduce war time decisions and activities for training purposes or to analyze the outcome of possible war time decisions. War games, however, can be much smaller than full-scale military operations, do not typically include the use of functional military equipment, and decisions and actions are carried out by artificial players to simulate possible decisions and actions within an artificial scenario which usually represents a model of a real-world scenario. Additionally, mathematical modeling is used in the simulation of war games to provide a quantifiable method of deduction. However, it is rare that a war game is depended upon for quantitative results, and the use of war games is more often found in situations where qualitative factors of the simulated scenario are needed to be determined.

The actual use of war games and the results that they can provide are limited by possibilities. War games cannot be used to achieve predictive results, as the nature of war and the scenarios that war games aim to simulate are not deterministic. Therefore, war games are primarily used to consider multiple possible outcomes of any given decision, or number of decisions, made in the simulated scenario. These possible outcomes are analyzed and compared, and cause-and-effect relationships are typically sought for the unknown factors within the simulation. It is typically the relationships between visual aspects of the simulation that aid in the assessment of the problems that are simulated within war games, like geographic locations and positionings that would be difficult to discern or analyze at full-scale and for complex environments.

Military exercises involving multiple branches of the same military are known as joint exercises, while military exercises involving two or more countries are known as combined, coalition, bilateral, or multilateral exercises, depending on the nature of the relationship between the countries and the number of them involved. These exercises allow for better coordination between militaries and observation of enemy tactics, and serve as a visible show of strength and cooperation for the participating countries. According to a 2021 study, joint military exercises within well-defined alliances usually deter adversaries without producing a moral hazard because of the narrow scope of the alliance, while joint military exercises outside of an alliance (which are extremely rare) usually lead to conflict escalation.

Exercises in the 20th and 21st centuries have often been identified by a unique code name, such as Cobra Gold, in the same manner as military contingency operations and combat operations like Operation Phantom Fury.

Military exercises are sometimes used as cover for the build up to an actual invasion, as in the cases of the Warsaw Pact invasion of Czechoslovakia and the 2022 Russian invasion of Ukraine, or it can provoke opponents at peace to perceive it as such, as in the case of Able Archer 83.