Secret treaty

A secret treaty is "an international agreement in which the contracting parties have agreed, either in the treaty instrument or separately, to conceal its existence or at least its substance from other states and the public."

According to one compilation of secret treaties published in 2004, there have been 593 secret treaties negotiated by 110 countries and independent political entities since the year 1521. "Secret treaties were a central instrument of balance-of-power diplomacy in the eighteenth and nineteenth centuries," but are rare today.