Modern Romania emerged within the territories of the ancient Roman province of Dacia, and was formed in 1859 through a personal union of the Danubian Principalities of Moldavia and Wallachia. The new state, officially named Romania since 1866, gained independence from the Ottoman Empire in 1877. At the end of World War I, Transylvania, Bukovina and Bessarabia united with the sovereign Kingdom of Romania. During World War II, Romania was an ally of Nazi Germany against the Soviet Union, fighting side by side with the Wehrmacht until 1944, when it joined the Allied powers and faced occupation by the Red Army forces. Romania lost several territories, of which Northern Transylvania was regained after the war. Following the war, Romania became a socialist republic and member of the Warsaw Pact.