The MolotovRibbentrop Pact was a non-aggression pact between Nazi Germany and the Soviet Union that enabled those two powers to partition Poland between them. The pact was signed in Moscow on 23 August 1939 by German Foreign Minister Joachim von Ribbentrop and Soviet Foreign Minister Vyacheslav Molotov and was officially known as the Treaty of Non-Aggression between Germany and the Union of Soviet Socialist Republics. Unofficially, it has also been referred to as the HitlerStalin Pact, NaziSoviet Pact or NaziSoviet Alliance (although it was not a formal alliance).The establishment of the treaty was preceded by Soviet efforts to form a tripartite alliance with Britain and France. Fearing encirclement by hostile powers, the Soviet Union began negotiations with Germany on 22 August, one day after talks broke down with Britain and France, and the MolotovRibbentrop pact was signed the next day. Its clauses provided a written guarantee of peace by each party towards the other and a commitment that declared that neither government would ally itself to or aid an enemy of the other. In addition to the publicly-announced stipulations of non-aggression, the treaty included the Secret Protocol, which defined the borders of Soviet and German spheres of influence across Poland, Lithuania, Latvia, Estonia and Finland. The secret protocol also recognised the interest of Lithuania in the Vilnius region, and Germany declared its complete disinterest in Bessarabia. The rumoured existence of the Secret Protocol was proved only when it was made public during the Nuremberg Trials.Soon after the pact, Germany invaded Poland on 1 September 1939. Soviet leader Joseph Stalin ordered the Soviet invasion of Poland on 17 September, one day after a SovietJapanese ceasefire came into effect after the Battles of Khalkhin Gol. After the invasions, the new border between the two countries was confirmed by the supplementary protocol of the GermanSoviet Frontier Treaty. In March 1940, parts of the Karelia and Salla regions, in Finland, were annexed by the Soviet Union after the Winter War. That was followed by the Soviet annexation of Estonia, Latvia, Lithuania and parts of Romania (Bessarabia, Northern Bukovina and the Hertsa region). Concern for ethnic Ukrainians and Belarusians had been used as pretexts for the Soviets' invasion of Poland. Stalin's invasion of Bukovina in 1940 violated the pact since it went beyond the Soviet sphere of influence that had been agreed with the Axis.The territories of Poland annexed by the Soviet Union after the 1939 Soviet invasion east of the Curzon line remained in the Soviet Union after the war ended and are now in Ukraine and Belarus. Vilnius was given to Lithuania. Only Podlaskie and a small part of Galicia east of the San River, around Przemyl, were returned to Poland. Of all the other territories annexed by the Soviet Union in 1939 to 1940, those detached from Finland (Western Karelia, Petsamo), Estonia (Estonian Ingria and Petseri County) and Latvia (Abrene) remain part of Russia, the successor state to the Russian SSR after the dissolution of the Soviet Union in 1991. The territories annexed from Romania had also been integrated into the Soviet Union (as the Moldavian SSR or oblasts of the Ukrainian SSR). The core of Bessarabia now forms Moldova. Northern Bessarabia, Northern Bukovina and the Hertsa region now form the Chernivtsi Oblast of Ukraine. Southern Bessarabia is part of the Odessa Oblast, which is also in Ukraine.

The pact was terminated on 22 June 1941, when Germany launched Operation Barbarossa and invaded the Soviet Union, in pursuit of the ideological goal of Lebensraum. After the war, Ribbentrop was convicted of war crimes at the Nuremberg trials and executed. Molotov died in 1986.

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.