The Red Republic is a rarely-seen nation of the plastic world. They appear as a playable faction in the multiplayer of Army Men: Sarge's War and Army Men: RTS, and one is rescued in Dr. Madd's castle in the PS1 version of Army Men: Sarge's Heroes 2. They also appear in the PS1 and PC version of Army Men: Sarge's Heroes, in the cut-scene where Shrap is rescued from the spider machine. It is possible that they represent a communist style army. It is also unknown where this Nation resides, though if it is anything like our world, they are probably east of the Blue Nation.
There is a fair chance that the invasion of Dryuun's castle was the source of the Red Republic, their name being synonymous to the Chinese Red Faction army. If this were the case however, then perhaps they have siege fortresses such as along the Hongo wall.