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.
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.
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.