The idea that resorting to war can only be just under certain conditions goes back at least to Cicero. However its importance is connected to Christian medieval theory beginning from Augustine of Hippo and Thomas Aquinas. 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. The importance of the theory of just war faded with revival of classical republicanism beginning with works of Thomas Hobbes.