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Concordats in the sense of a "treaty" were prominent in Pope Pius XI's papacy (cf. the Lateran Treaty) and in Pope Pius XII's also (Pope Pius XII was Pope Pius XI's secretary of state). See this list of concordats. Also, there are some good books on the Catholic Church and State doctrine: Catholic Church and Christian State: A Series of Essays on the ...
Found it in this question: What exactly did Pope Gregory the Great mean by “Universal Bishop?” Pope St. Gregory the Great, 6th century, from his epistles.
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