Russia is the main invasion force, with Cuban and central American auxiliary units. It’s explained in the film by Col Tanner, and he also detailed the nuclear exchange by the Soviets.
The central American forces/soldiers were the REMF forces needed to occupy key rear areas freeing up the Red Army to fight on the “stalled” front. In fact, the town that the story takes place was a key rail/ammo storage facility occupied by Nicaraguan and Soviet special forces.