If I may...
(Yes I know it's Wikipedia...but it's a place to start
wikipedia: Just War
The idea that resorting to war can only be just under certain conditions goes back at least to Cicero.[3] However its importance is connected to Christian medieval theory beginning from Augustine of Hippo[4] and Thomas Aquinas.[5] The first work dedicated specifically to it was De bellis justis of Stanisław of Skarbimierz, who justified war of the Kingdom of Poland with Teutonic Knights. Francisco de Vitoria justified conquest of America by the Kingdom of Spain. With Alberico Gentili and Hugo Grotius just war theory was replaced by international law theory, codified as a set of rules, which today still encompass the points commonly debated, with some modifications.[citation needed] The importance of the theory of just war faded with revival of classical republicanism beginning with works of Thomas Hobbes.
The Just War Theory is an authoritative Catholic Church teaching confirmed by the United States Catholic Bishops in their pastoral letter, The Challenge of Peace: God's Promise and Our Response, issued in 1983. More recently, the Catechism of the Catholic Church, in paragraph 2309, lists four strict conditions for "legitimate defense by military force"
(snip)
Point is The Just War Theory goes back a lot farther than Reinhold Niebuhr.
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