Is violence ever an acceptable means to an end? This question has troubled mankind for millennia.As early as the first century B.C., Rome's Marcus Tullius Cicero (106-43 B.c.) theorized on the justified use of armed force, a concept still debated by scholars and political leaders today.Modern just war theory coalesced in the fifth century with the teachings of St. Augustine of Hippo (c.c. 354-), a founder of Western theology. In The City of God, published in A.D. 426, he explored the inherent conflict between Christianity and violence. Augustine concluded that, although wars of aggression were never acceptable, sometimes war was a necessity for self-defense or protection of the innocent. He posited that in order to be considered just, warring parties must not target neutral parties so-called noncombatant immunity, which was invoked by Switzerland in World War II.Nine hundred years later, Italian philosopher Thomas Aquinas (1225-1274) built on this theory, describing standards that are still used to define a "just war": The war must be openly declared by an appropriate authority such as a recognized state; it must have a just cause; and the ultimate goal must be to establish a just peace. Despite the to avoid unnecessary violence, "just war" has been invoked as a battle cry for conflicts from the Crusades to the United States' 2003 Iraq invasion.