Clans of the Alphane Moon - Philip K. Dick

For years, the third moon in the Alphane system was used as a psychiatric hospital. But when war broke out between Earth and the Alphanes, the hospital was left unguarded and the inmates set up their own society, made up of competing factions based around each mental illness. When Earth sends a delegation to take back the colony, they find enclaves of depressives, schizophrenics, paranoiacs, and other mentally ill people coming together to repel what they see as a foreign invasion.