An opposing force (alternatively enemy force, abbreviated OPFOR or OpFor) 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.

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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 replicate 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.)

• 49th Royal Tank Regiment Matilda II with swastika and Iron Cross markings to denote opposing force status during an 1941 exercise in Dover.
• Dutch soldiers of 11 Pantserinfanteriebataljon Garderegiment Grenadiers wearing mohawk-style additions on their helmets to denote opposing force status.
• New Zealand Defence Force soldier denoting opposing force status by wearing Desert Disruptive Pattern Material (DPM) in a temperate environment.
• Scene from a simulated Royal Marines beach landing during the 2009 Bournemouth Air Festival; "opposing force" Marine is bareheaded and wears Desert DPM, "blue force" Marines in the background wear berets, camouflage face paint, and woodland DPM.
• Royal Marine "opposing force" members during a 2016 assault demonstration; they wear Tropentarn uniforms and soft hats while the assaulting "blue force" Marines would be wearing MTP uniforms with Mk 7 helmets.
• A sniper from the French CENZUB opposing force wearing a blue-and-gray version of Camouflage Central-Europe.
• 51st Fighter Wing airman denoting opposing force status by wearing his Battle Dress Uniform back to front (note the contrast of the uniform with the Individual Integrated Fighting System vest that is also being worn).
• 4th Infantry Regiment soldiers wearing United States Army opposing force uniform (Battle Dress Uniform in black with olive 'OPFOR' shoulder title)
• An OSV replicating a Soviet BMP at NTC, Fort Irwin, CA
• US Marines using a former Soviet MT-LB vehicle as part of the OPFOR during an exercise
• Michigan National Guard OPFOR wearing tiger stripe camouflage uniform and carrying a simulated RPG-7.
• OPFOR rank insignia mimicking the Soviet style (used during the Cold War)