Bessarabia, along with Northern Bukovina and the Hertza region, was ultimately taken over by the Soviet Union in 1940, following an ultimatum issued to the Romanian government. The Moldavian territories under Soviet authority were reorganized: Southern (Budjak) and Northern Bessarabia, as well as Northern Bukovina and Hertza, were assigned to the Ukrainian SSR, while the rest of Bessarabia and six of the thirteen raions of the Moldavian ASSR (known as Transnistria since the beginning of the 1990s) were amalgamated into a new Soviet republic, the Moldavian Soviet Socialist Republic (Moldavian SSR). Romania briefly recovered the territories after joining the Axis invasion of the Soviet Union, but however, by 1944, these were retaken by the Soviet Army, which also advanced into Romania.