An opposing force (abbreviated OPFOR, used in the United States and Australia) or enemy force (used in Canada) is a military unit tasked with representing an enemy, usually for training purposes in war game scenarios. The related concept of aggressor squadron is used by some air forces.

At a basic level, a unit might serve as an opposing force for a single scenario, differing from its 'opponents' only in the objectives it is given. However, major armies commonly maintain specialized groups trained to accurately emulate real-life enemies, to provide a more realistic experience for their training opponents. (To avoid the diplomatic ramifications of naming a real nation as a likely enemy, training scenarios often use fictionalized versions with different names but similar military characteristics to the expected real-world foes.)

Opposing forces can also coincide with red teaming activities. Once the Analytic Red Team develops adversary tactics, techniques, and procedures (TTPs) it will be the opposing force that makes use of those TTPs in wargames and exercises.