The Rats of Nam Yum were internal deserters from the French forces at the Battle of Dien Bien Phu during the First Indochina War.
The "Rats" consisted of French, Foreign Legion African, Viet, and T'ai tribal troops who were unable or unwilling to defect to the Viet Minh opposing the French, but unwilling to fight for the French forces. They sequestered themselves in a portion of the fortress of Dien Bien Phu on the cliffs overlooking the Nam Yum river to wait out the battle.
The Rats subsisted on rations and equipment stolen from the French forces or from airdropped supplies they were able to intercept. Lacking shelters or building supplies, they took to digging dugouts for shelter.
References
Rats of Nam Yum Wikipedia(Text) CC BY-SA