Bessarabia is a region between the Prut and Dniester rivers that the Russian Empire annexed from the Principality of Moldavia in 1812. This led to a process of Russification and colonization of the province, which severely reduced the Romanian population of the region. In 1859, Moldavia united with Wallachia, forming the modern state of Romania. The beginning of the 1917 Russian Revolution allowed Bessarabia to declare itself as independent, forming the Moldavian Democratic Republic while World War I was taking place. On 27 March 1918, this state united with Romania, this being the reason for the commemorated date. Romania lost control of Bessarabia and also of Northern Bukovina after the occupation of these territories by the Soviet Union (USSR) in 1940, with Romania regaining them briefly in 1941 but losing them again in 1944.