The most influential early Christian thinker to wrestle with these issues was the North African bishop St Augustine of Hippo (354–430 CE). His work laid the foundation upon which the papacy eventually built the notion of crusading. St Augustine argued that a war could be both lawful and justifiable if fought under strict conditions. His complex theories were later simplified to produce just three prerequisites of a Just War: proclamation by a ‘legitimate authority’, such as a king or bishop; a ‘just cause’, like defence against enemy attack or the recovery of lost territory; and prosecution
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