This paper seeks to investigate how the mutual guarantee clauses of the treaties of Westphalia, which ended the Thirty Years War in 1648, affected European diplomacy until the late eighteenth-century. It will firstly analyse the reception and impact of the guarantee of the Peace of Westphalia in the European Law of Nations and in subsequent treaty law. Secondly, it will assess the practical impact of this feature of the Law of Nations on European diplomacy, and how this influence changed over time. This will also include an analysis of how diplomacy and shifting power-political currents altered the content of the guarantee in the Law of Nations. In analysing the guarantee's influence on diplomacy, the paper places a particular emphasis on Franco-Imperial and Swedish-Imperial relations, as well as the perception of the guarantee among diplomats and other political actors during political, constitutional and confessional conflicts within the Holy Roman Empire.