The land of what is now modern Estonia has been inhabited since at least 9,000 BC. Ancient Estonians were one of the last "pagan" civilisations in Europe to adopt Christianity following the Livonian Crusade in the 13th century. After centuries of successive rule by the Teutonic Order, Denmark, Poland, Sweden, and the Russian Empire, a distinct Estonian national identity began to emerge by the end of the 19th century. This culminated in the 1918 Estonian Declaration of Independence from Russia and German Empire and, after the end of World War I, in the 1918-1920 War of Independence, where Estonians were able to repel the Soviet Russian invasion and successfully defended their newborn freedom. Democratic throughout most of the interwar period, after the onset of worldwide Great Depression Estonia also experienced authoritarian rule during the politically non-violent "era of silence" in 1934-1938. During World War II, Estonia was repeatedly contested, invaded and occupied by Germany and the Soviet Union, ultimately being incorporated into the latter as the Estonian SSR. After the loss of its ''de facto'' independence to the Soviet Union, Estonia's ''de jure'' state continuity was preserved by diplomatic representatives and the government-in-exile. After the peaceful "Singing Revolution" of 1988-1990, the nation's ''de facto'' independence was restored on 20 August 1991.