Oldenburg attained its independence from Denmark in 1773 and in 1777 was elevated to the status of duchy. Thus, when the United States announced its independence from Great Britain in 1776, Oldenburg was a sovereign, independent state. Annexed by France in 1810, the Duchy of Oldenburg was restored and elevated to the status of Grand Duchy following the 1815 Congress of Vienna. The United States and the Grand Duchy of Oldenburg mutually recognized each other in 1829.
During the 1866 Austro-Prussian War, Oldenburg sided with Prussia. In 1867 Oldenburg was incorporated into the North German Confederation, after which point foreign policy for the new confederation was conducted out of Berlin. Oldenburg, like the other North German Confederation members, joined the German Empire in 1871. From this point forward, foreign policy of the German Empire was made in Berlin, with the German Kaiser (who was also the King of Prussia) accrediting ambassadors of foreign nations. Relations with Imperial Germany were severed when the U.S. declared war in 1917.
The first known act of mutual recognition between the United States and the Grand Duchy of Oldenburg was on December 2, 1829, when U.S. Secretary of State Martin Van Buren issued an exequatur to Frederick A. Mensch Esq. as Consul for the Grade Duke of Oldenburg at New York.
The United States and the Grand Duchy of Oldenburg never established diplomatic relations.
On March 10, 1847, the United States and the Grand Duchy of Oldenburg signed the Declaration of Accession to the Treaty of Commerce and Navigation with Hanover, to regulate trade, commerce, and navigation between the U.S. and Oldenburg. The declaration was signed by U.S. Special Agent A.D. Mann and Oldenburg’s head of Foreign Affairs, Baron W.E. de Beaulieu Marconnay.
On December 30, 1853, the Grand Duchy of Oldenburg signed the Declaration of Accession to the Convention for the Extradition of Criminals, Fugitives from Justice, of June 16, 1852 Between the United States and Prussia and Other States of the Germanic Confederation, to establish reciprocal extradition of fugitive criminals in special cases.
The Duchy of Oldenburg joined the German Empire, which was proclaimed on January 18, 1871.
The Duchy of Oldenburg was one of the states involved in the process of German unification during the mid-nineteenth century. (See “Unification of German States” for greater detail.)