The city has a complex recorded history, partially due to the combined regional importance of the Port of Klaipėda, a usually ice-free port on the Baltic Sea, and the Akmena - Dange River. It has been controlled by the Teutonic Knights, the Duchy of Prussia, the Kingdom of Prussia, the German Empire, the Entente States immediately after World War I, Lithuania as a result of the 1923 Klaipėda Revolt, and the Third Reich following the 1939 German ultimatum to Lithuania. The city was incorporated into Lithuania during its tenure as a Soviet Socialist Republic and has remained within Lithuania following its re-establishment as an independent state.