Wartime collaboration is the voluntary cooperation of local populations, officials or combatants with the enemy against their own state, often for personal gain, ideology or survival. It is widely considered as a form of betrayal and a crime (treason) as it is viewed as a breach of loyalty that compromises the safety of the citizens and sovereignty of the state. As historian Gerhard Hirschfeld says, it "is as old as war and the occupation of foreign territory".