Most favoured nation (mfn) clauses, extending the benefits of the most favoured nation to the treaty partner, have existed since at least the 11th century. From the 18th century on, mfn clauses were frequently included in trade agreements. The widespread use of the mfn principle has furthered the equal treatment of nations and created prosperity and peace. In the multilateral framework of the World Trade Organization (wto), the mfn principle is a cornerstone, with only few exceptions to it. This article reviews the development of the mfn clause in major trade agreements in relation to historical events from 1648 onwards. The aim of this study is to determine what the consequences of protectionism, measured by unequal treatment of trading partners and protectionist policies has been in the past.