Combined arms is an approach to warfare that seeks to integrate different combat arms of a military to achieve mutually complementary effects—for example, using infantry and armour in an urban environment in which each supports the other.

According to the strategist William S. Lind, combined arms can be distinguished from the concept of "supporting arms" as follows:

Combined arms hits the enemy with two or more arms simultaneously in such a manner that the actions he must take to defend himself from one make him more vulnerable to another. In contrast, supporting arms is hitting the enemy with two or more arms in sequence, or if simultaneously, then in such combination that the actions the enemy must take to defend himself from one also defends himself from the other(s).

Though the lower-echelon units of a combined arms team may be of similar types, a balanced mixture of such units are combined into an effective higher-echelon unit, whether formally in a table of organization or informally in an ad hoc solution to a battlefield problem. For example, an armoured division, the modern paragon of combined arms doctrine, consists of a mixture of mechanized infantry, tank, artillery, reconnaissance, anti-air support, drone support, close air support and helicopter units, all of which are co-ordinated and directed by a unified command structure.

Also, most modern military units can, if the situation requires it, call on yet more branches of the military, such as infantry requesting space warfare, Cyberwarfare or Electromagnetic warfare support; or the bombing or shelling by military aircraft or naval forces to augment their ground offensive or protect their land forces. The mixing of arms is sometimes pushed down below the level at which homogeneity ordinarily prevails, such as by temporarily attaching a tank company to an infantry battalion.

Combined arms operations date back to antiquity, where armies would usually field a screen of skirmishers to protect their spearmen during the approach to contact. Especially in the case of the Greek hoplites, however, the focus of military thinking lay almost exclusively on the heavy infantry. In more elaborate situations armies of various nationalities fielded different combinations of light, medium, or heavy infantry, light or heavy cavalry, chariotry, camelry, elephantry, and artillery (mechanical weapons). Combined arms in this context was how to best use the cooperating units, variously armed with side-arms, spears, or missile weapons in order to coordinate an attack to disrupt and then destroy the enemy.

Philip II of Macedon greatly improved upon the limited combined arms tactics of the Greek city-states and combined the newly created Macedonian phalanx with heavy cavalry and other forces. The phalanx would hold the opposing line in place, until the heavy cavalry could smash and break the enemy line by achieving local superiority.

The early Republic Roman Legion was a combined arms force and consisted of five classes of troops. Lightly equipped velites acted as skirmishers armed with light javelins. The hastati and principes formed the main attacking strength of the legion with swords and pila, whilst the triarii formed the defensive backbone of the legion fighting as spearmen (initially as a denser Greek phalanx and later as a looser spear wall formation) with long spears and large shields. The fifth class of troops were the equites (the cavalry), which were used for scouting, pursuit and to guard the flanks.