Upon annexation, Romanian population of Bessarabia was predominant. The colonization of the region in the 19th century lead to a large increase of Russian, Ukrainian, Lipovan, and Cossack populations in the region; this together with a large influx of Bulgarian immigrants, saw an increase of the Slavic population to more than a fifth of the total population by 1920. With the settling of other nationals such as Gagauz, Jews, and Germans, the proportion of the Romanian population decreased from cca. 90% to 64% during the course of the century. The Tsarist policy in Bessarabia was in part aimed at denationalization of the Romanian element by forbidding after the 1860s education and mass in Romanian. However, the effect was an extremely low literacy rate (in 1897 approx. 18% for males, approx. 4% for females) rather than a denationalization. Some Romanian historians claimed that a strong sentiment of frustration and resentment to the Russian control had started to appear before the beginning of the World War I.
World War I brought in a rise in political and cultural (national) awareness of the locals, as 300,000 Bessarabians enrolled in the Russian Army formed in 1917, within bigger units several "Moldavian Solders' Committees". Following the Russian Revolution of 1917, Bessarabia elected its own parliament, Sfatul Ţării (October-November 1917), which opened on December 3, 1917, proclaimed the Moldavian Democratic Republic (December 15, 1917), formed its government (December 21, 1917), proclaimed independence from Russia (February 6, 1918), and on April 9, 1918, Sfatul Ţării decided with 86 votes for, 3 against and 36 abstaining, towards the union with the Kingdom of Romania, conditional upon the fulfillment of the agrarian reform, local autonomy, and respect for universal human rights.
The county councils of Bălţi, Soroca and Orhei were the earliest to ask for unification with the Kingdom of Romania, and on April 9 [O.S. March 27] 1918, Sfatul Ţării voted in favour of the union, with the following conditions:
In the autumn of 1919, elections for the Romanian Constituent Assembly were held in Bessarabia; 90 deputies and 35 senators were chosen. On December 20, 1919, these men voted, along with the representatives of Romania's other regions, to ratificaty the unification acts that had been approved by Sfatul Ţării and the National Congresses in Transylvania and Bukovina.
The union was recognized by the European countries in the Treaty of Paris (1920). The newly-communist Russia was not represented as a party at the treaty conference. A mutual treaty between the Soviets and Romania was not signed due to the former's claims over Bessarabia. In the Kellogg-Briand Treaty of 1928 and the Treaty of London of July 1933, the Soviet Union and Romania have subscribed to the principle of non-violent resolution of territorial disputes. Transnistria, at the time part of the Ukrainian SSR, itself part of the USSR, was formed into the Moldavian ASSR (1924-1940) after the failure of the Tatarbunar uprising.
The agrarian (land) reform, settled by Sfatul Tarii in 1918-1919, resulted in a rise of a middle class, as the rural population of the region represented 80%. Together with peace and favorable economic circumstances, it produced a small economic boom, which allowed the region to catch up technologically with the rest of Europe. A lawful land reform, promoted by the majority of the political leadership of Bessarabia in 1918, as opposed to an irregular expropriation of property, promoted by the pro-Soviet elements, was more appealing to the local farmers, and is at least partly responsible for the consent the peasantry has given to the intelligentsia's plans for building a unified state for all Romanians. The literacy rate grew to over 40% by 1930, however the region still remained lagging in the aspect of education. In an attempt to alienate the Bessarabian ethnic minorities from the Russian influence, the Romanian authorities have allowed education in any language desired; with time, while Romanian replaced Russian in cities, the authorities sought to reduce the number of ethnic minority education and attract them into Romanian classes.
The official Soviet policy also stated that Romanian and Moldovan were two different languages and, to emphasize the distinction, Moldovan was written using a special Cyrillic alphabet (the Moldovan alphabet) derived from the Russian alphabet – unlike Romanian, written with its own version of the Latin alphabet.