5 And the ties that consecrated this union should have been doubly inviolable from the fact that they were sanctioned by sworn treaties. The Concordat entered upon by the Sovereign Pontiff and the French Government was, like all treaties of the same kind concluded between States, a bilateral contract binding on both parties to it. The Roman Pontiff on the one side and the Head of the French Nation on the other solemnly stipulated both for themselves and their successors to maintain inviolate the pact they signed. Hence the same rule applied to the Concordat as to all international treaties, viz., the law of nations which prescribes that it could not be in any way annulled by one alone of the contracting parties. The Holy See has always observed with scrupulous fidelity the engagements it has made, and it has always required the same fidelity from the State. This is a truth which no impartial judge can deny. Yet today the State, by its sole authority, abrogates the solemn pact it signed. Thus it violates its sworn promise. To break with the Church, to free itself from her friendship, it has stopped at nothing, and has not hesitated to outrage the Apostolic See by this violation of the law of nations, and to disturb the social and political order itself - for the reciprocal security of nations in their relations with one another depends mainly on the inviolable fidelity and the sacred respect with which they observe their treaties.
Source: Vehementer Nos (Vatican.va)