The Soviet Union had planned to accomplish the annexation with a full-scale invasion, but the Romanian government, responding to a Soviet ultimatum delivered on June 26, agreed to withdraw from the territories in order to avoid a military conflict. Germany, which had acknowledged the Soviet interest in Bessarabia in a secret protocol to the 1939 Molotov-Ribbentrop Pact, had been made aware prior to the planned ultimatum on June 24, but had not informed the Romanian authorities, nor were they willing to provide support. The Fall of France, a guarantor of Romania's borders, on 22 June, is considered an important factor in the Soviet decision to issue the ultimatum.
On August 2, 1940, the Moldavian Soviet Socialist Republic was proclaimed as a constituent republic of the Soviet Union, encompassing most of Bessarabia, as well as a portion of the Moldavian Autonomous Soviet Socialist Republic, an autonomous republic of the Ukrainian SSR located on the left bank of the Dniester (now the breakaway Transnistrian state). The Hertza region, and the regions inhabited by Slavic majorities (Northern Bukovina, Northern and Southern Bessarabia) were included in the Ukrainian SSR. A period of political persecution, including executions, deportations to labour camps and arrests, occurred during the Soviet administration.
In July 1941, Romanian and German troops recaptured Bessarabia during the Axis invasion of the Soviet Union. A military administration was established and the region's Jewish population was either executed on the spot or deported to Transnistria, where further numbers were killed. In August 1944, during the Soviet Jassy-Kishinev Offensive, the Axis war effort on the Eastern Front collapsed. A coup d'état in Romania on 23 August 1944 caused the Romanian army to cease resisting the Soviet advance and join the fight against Germany. Soviet forces advanced from Bessarabia into Romania, capturing much of its standing army as POWs and occupying the country. On September 12, 1944, Romania signed the Moscow Armistice with the Allies. The Armistice, as well as the subsequent peace treaty of 1947, confirmed the Soviet-Romanian border as it was on January 1, 1941.
Bessarabia, Northern Bukovina, and Hertza remained part of the Soviet Union until its dissolution in 1991, when they became part of the newly independent states of Moldova and Ukraine. In its declaration of independence of August 27, 1991, the government of Moldova condemned the creation of the Moldavian SSR, declaring that it had no legal basis.
As a historical region, Bessarabia was the eastern part of the Principality of Moldavia. In 1812, under the terms of the Treaty of Bucharest, the region was ceded by the Ottoman Empire (to which Moldavia was a vassal state) to the Russian Empire.