They’re the units that everyone wants to beat, that every commander wants to squash under their heel, and that most average Joes accuse of cheating at least once — the “Opposing Forces” units at military training centers.

The OPFOR units are comprised of active-duty soldiers stationed at major training centers and are tasked with playing enemy combatants in training exercises for the units that rotate into their center. They spend years acting as the adversary in every modern training exercise their base can come up with.

The "most hated units" in the Army are some of the best
American Army Pfc. Sean P. Stieren, a rifleman with A Company, 1st Battalion, 509th Infantry Regiment (Airborne), fires a mock Stinger missile launcher at the Joint Readiness Training Center in Fort Polk, La., April 19, 2014. Paratroopers with 1st Bn., 509th Inf. Reg. (A), act as insurgent and conventional opposing forces during decisive action training environment exercises at JRTC. (Photo by Staff Sgt. Christopher Klutts/U.S. Army)

So while most units do a rotation at a major training center every couple of years, soldiers assigned to OPFOR units often conduct major training rotations every month. This results in their practicing the deployed lifestyle for weeks at a time about a dozen times per year.

Through all this training, they get good. Really good.

And since they typically conduct their missions at a single installation or, in rare cases, at a few training areas in a single region, they’re experts in their assigned battlespace.

The "most hated units" in the Army are some of the best.
A U.S. Army soldier with 1st Battalion, 509th Infantry Regiment (Airborne) fires blank rounds at soldiers from a rotational training unit during an exercise at the Joint Readiness Training Center in Fort Polk, La., April 22, 2014. Paratroopers with 1st Bn., 509th Inf. Reg. (A) role-play as multiple enemy forces including a near-peer military, insurgent cells, and a crime family. (Photo by Staff Sgt. Christopher Klutts/U.S. Army)

All this adds up to units with lots of experience against the best units the military has to deploy — units that are at the cutting edge of new tactics, techniques, and procedures; units that have the home-field advantage.

“The first time you fight against the OpFor is a daunting experience,” Maj. Jared Nichols, a battalion executive officer that rotated through the National Training Center at Fort Irwin, California, said during a 2016 training iteration. “You’re fighting an enemy that knows the terrain and knows how American forces fight, so they know how to fight against us and they do it very well.”

So yeah, despite typically fighting at a 2-to-1 or even a 3-to-1 disadvantage, OPFOR units often decimate their opponents.