The Soviet occupation of Bessarabia and Northern Bukovina took place from June 28 to July 3, 1940, as a result of an ultimatum by the Soviet Union to Romania on June 26, 1940, that threatened the use of force. Bessarabia had been part of the Kingdom of Romania since the time of the Russian Civil War and Bukovina since the dissolution of Austria-Hungary, and Hertsa was a district of the Romanian Old Kingdom. Those regions, with a total area of 50,762 km2 (19,599 sq mi) and a population of 3,776,309 inhabitants, were incorporated into the Soviet Union. On October 26, 1940, six Romanian islands on the Chilia branch of the Danube, with an area of 23.75 km2 (9.17 sq mi), were also occupied by the Soviet Army.The Soviet Union had planned to accomplish the annexation with a full-scale invasion, but the Romanian government, responding to the Soviet ultimatum delivered on June 26, agreed to withdraw from the territories to avoid a military conflict. The use of force had been made illegal by the Conventions for the Definition of Aggression in July 1933, but from an international legal standpoint, the new status of the annexed territories was eventually based on a formal agreement through which Romania consented to the retrocession of Bessarabia and cession of Northern Bukovina. As it was not mentioned in the ultimatum, the annexation of the Hertsa region was not consented to by Romania, and the same is true of the subsequent Soviet occupation of the Danube islands. On June 24, Nazi Germany, which had acknowledged the Soviet interest in Bessarabia in a secret protocol to the 1939 MolotovRibbentrop Pact, had been made aware prior to the planned ultimatum but did not inform the Romanian authorities and was willing to provide support. On June 22, France, a guarantor of Romanian borders, fell to Nazi advances. This is considered to be an important factor in the Soviets' decision to issue the ultimatum. The Soviet invasion of Bukovina in 1940 violated the Molotov-Ribbentrop Pact, since it went beyond the Soviet sphere of influence that had been agreed with the Axis.

On August 2, 1940, the Moldavian Soviet Socialist Republic was proclaimed as a constituent republic of the Soviet Union, encompassing most of Bessarabia and part of the Moldavian Autonomous Soviet Socialist Republic, an autonomous republic of the Ukrainian Soviet Socialist Republic on the left bank of the Dniester (now the breakaway Transnistria). The Hertsa 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 occupied Bessarabia, Northern Bukovina and Hertsa during the Axis invasion of the Soviet Union. A military administration was established, and the region's Jewish population was executed on the spot or deported to Transnistria, where large numbers were killed. In August 1944, during the Soviet Second JassyKishinev Offensive, the Axis war effort on the Eastern Front collapsed. The coup of 23 August 1944 caused the Romanian army to cease resisting the Soviet advance and to join the fight against Germany. Soviet forces advanced from Bessarabia into Romania, captured much of its standing army as prisoners-of-war and occupied the country. On September 12, 1944, Romania signed the Moscow Armistice with the Allies. The Armistice and the subsequent peace treaty of 1947 confirmed the Soviet-Romanian border as it was on January 1, 1941.Bessarabia, Northern Bukovina and Hertsa remained part of the Soviet Union until it collapsed in 1991, when they became part of the newly independent states of Moldova and Ukraine. The Declaration of Independence of Moldova of August 27, 1991, declared the Soviet occupation illegal.

World War II or the Second World War, often abbreviated as WWII or WW2, was a global war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing military alliances: the Allies and the Axis powers. In a total war directly involving more than 100 million personnel from more than 30 countries, the major participants threw their entire economic, industrial, and scientific capabilities behind the war effort, blurring the distinction between civilian and military resources. Aircraft played a major role in the conflict, enabling the strategic bombing of population centres and the only two uses of nuclear weapons in war. World War II was by far the deadliest conflict in human history; it resulted in 70 to 85 million fatalities, a majority being civilians. Tens of millions of people died due to genocides (including the Holocaust), starvation, massacres, and disease. In the wake of the Axis defeat, Germany and Japan were occupied, and war crimes tribunals were conducted against German and Japanese leaders.

The exact causes of World War II are debated, but contributing factors included the Second Italo-Ethiopian War, the Spanish Civil War, the Second Sino-Japanese War, the Soviet–Japanese border conflicts and rising European tensions since World War I. World War II is generally considered to have begun on 1 September 1939, when Nazi Germany, under Adolf Hitler, invaded Poland. The United Kingdom and France subsequently declared war on Germany on 3 September. Under the Molotov–Ribbentrop Pact of August 1939, Germany and the Soviet Union had partitioned Poland and marked out their "spheres of influence" across Finland, Estonia, Latvia, Lithuania and Romania. From late 1939 to early 1941, in a series of campaigns and treaties, Germany conquered or controlled much of continental Europe, and formed the Axis alliance with Italy and Japan (along with other countries later on). Following the onset of campaigns in North Africa and East Africa, and the fall of France in mid-1940, the war continued primarily between the European Axis powers and the British Empire, with war in the Balkans, the aerial Battle of Britain, the Blitz of the UK, and the Battle of the Atlantic. On 22 June 1941, Germany led the European Axis powers in an invasion of the Soviet Union, opening the Eastern Front, the largest land theatre of war in history.

Japan, which aimed to dominate Asia and the Pacific, was at war with the Republic of China by 1937. In December 1941, Japan attacked American and British territories with near-simultaneous offensives against Southeast Asia and the Central Pacific, including an attack on the US fleet at Pearl Harbor which resulted in the United States declaring war against Japan. Therefore the European Axis powers declared war on the United States in solidarity. Japan soon captured much of the western Pacific, but its advances were halted in 1942 after losing the critical Battle of Midway; later, Germany and Italy were defeated in North Africa and at Stalingrad in the Soviet Union. Key setbacks in 1943—including a series of German defeats on the Eastern Front, the Allied invasions of Sicily and the Italian mainland, and Allied offensives in the Pacific—cost the Axis powers their initiative and forced it into strategic retreat on all fronts. In 1944, the Western Allies invaded German-occupied France, while the Soviet Union regained its territorial losses and turned towards Germany and its allies. During 1944 and 1945, Japan suffered reversals in mainland Asia, while the Allies crippled the Japanese Navy and captured key western Pacific islands.

The war in Europe concluded with the liberation of German-occupied territories, and the invasion of Germany by the Western Allies and the Soviet Union, culminating in the fall of Berlin to Soviet troops, Hitler's suicide and the German unconditional surrender on 8 May 1945. Following the Potsdam Declaration by the Allies on 26 July 1945 and the refusal of Japan to surrender on its terms, the United States dropped the first atomic bombs on the Japanese cities of Hiroshima, on 6 August, and Nagasaki, on 9 August. Faced with an imminent invasion of the Japanese archipelago, the possibility of additional atomic bombings, and the Soviet’s declared entry into the war against Japan on the eve of invading Manchuria, Japan announced on 15 August its intention to surrender, then signed the surrender document on 2 September 1945, cementing total victory in Asia for the Allies.

World War II changed the political alignment and social structure of the globe. The United Nations (UN) was established to foster international co-operation and prevent future conflicts, with the victorious great powers—China, France, the Soviet Union, the United Kingdom, and the United States—becoming the permanent members of its Security Council. The Soviet Union and the United States emerged as rival superpowers, setting the stage for the nearly half-century-long Cold War. In the wake of European devastation, the influence of its great powers waned, triggering the decolonisation of Africa and Asia. Most countries whose industries had been damaged moved towards economic recovery and expansion. Political and economic integration, especially in Europe, began as an effort to forestall future hostilities, end pre-war enmities and forge a sense of common identity.