According to Abbreviationfinder, USSR stands for Union of Soviet Socialist Republics.
The USSR and World War II
Once the Second World War began, however, and considering Hitler that the fall of England was imminent, he ordered an attack on the Soviet Union, making the pact a dead letter. The 18 of December of 1940, the German command decided that the invasion of the Soviet Union (Operation Barbarossa) would be held in April 1941, but could only realize the June 22 of that year, when the attack on Soviet territory began with more than 3,000,000 German soldiers. The invasion took Stalin by surprise even though he had a sufficient record from various sources of his own intelligence (such as Soviet agent Richard Sorge) that it was imminent. From the beginning of the invasion, the war received, on the part of the Soviet people, the name of Great Patriotic War or Great Patriotic War.
Initially the German forces advanced rapidly through the western plains of the USSR, causing immense losses in human and material resources to the Red Army. However, the Soviet resistance failed the attempts to take Leningrad and Moscow, the latter in November-December 1941. The non-occupied part of the country was transformed into a production zone continues to ensure resistance and victory, while in the Nazi-occupied areas burned with guerrilla resistance.
The Red Army stopped the Nazi offensive at the Battle of Stalingrad, from late 1942 to early
Parade of the Red Army after the Soviet victory over Nazi Germany.
of 1943, to later damage him definitively in the Battle of Kursk, being the major decisive point, and he advanced through Eastern and Central Europe to Berlin until the taking of Berlin and the unconditional surrender of Germany on May 8, 1945. 
Later, the USSR was a decisive part in the defeat of militaristic Japan and the liberation of northern China and the Korean peninsula. Although ravaged by war, the USSR emerged from the conflict as a recognized superpower and enormous prestige, having been the country that withstood the attack of 80% of the German forces and their allies, suffering more than 27 million casualties, among civilians and military.
During the immediate postwar period, the Soviet Union first rebuilt and then expanded its economy. The Soviet Union helped post-war rebuilding in Eastern European countries, founded the Warsaw Pact in 1955, later the Council for Mutual Economic Aid, helped emerging popular China economically and saw its influence grow elsewhere. of the world. Meanwhile, imperialist Cold War policy turned the Soviet Union’s wartime allies, the United Kingdom and the United States, as enemies.
Joseph Stalin died on March 5, 1953. After his death, his successor, Nikita Khrushchev, presented to the plenary session of the XX Congress of the Communist Party in 1956, a report with the political errors and crimes committed by Stalin, lamenting the cult of his personality and initiating a de-Stalinization campaign.
The Soviet Union unleashed enormous scientific and technological potential, launching the first artificial satellite Sputnik 1, the first living being to travel to space is Laika, and later, the first human being to orbit the Earth, Yuri Gagarin.
Valentina Tereshkova was the first woman to fly in space aboard Vostok 6 on 16 of June of 1963, and Alexei Leonov became the first person to walk in space on 18 of March of 1965. Khrushchev was retired in 1964. The enormous effort made to achieve nuclear parity with the United States contributed to bleeding the Soviet economy and together with other errors caused industry and agriculture to stagnate.
After Khrushchev, another period of rule by the Committee or collective command followed which lasted until Leonid Brezhnev established himself in the early 1970s as the pre-eminent figure in Soviet political life.
In the sports field it became the first world power, the Soviet Union organized the 1980 Olympic Games, based in Moscow. There was a boycott of the event by the United States: in the context of the Cold War and in protest at the entry of Soviet troops into Afghanistan, the Americans decided not to attend the Olympic Games, while trying to persuade their allies to that they did not attend either. In all, 65 countries abstained from participating, mainly due to US pressure.
Gorbachev’s reforms and the dissolution of the USSR
Gorbachev with Reagan signing the INF Treaty, in Washington DC, in 1987.
After the death of Leonid Brezhnev and after the rapid succession of Yuri Andropov and Konstantin Chernenko, Mikhail Gorbachev was appointed leader of the USSR. Gorbachev began to apply significant changes in the economy, Perestroika and Glásnost politics, unleashing opportunistic forces that with the encouragement of the West worked to disintegrate the USSR and the return of its members – especially Russia – to capitalism. The distancing of the Communist Party and its leadership from the workers favored this process.
The movement that definitively brought down the USSR came from Russia, the nation that had built the Tsarist empire, predecessor of the Soviet state. In May 1990, Borís Yeltsin, who had been expelled from the CPSU in 1987, was elected president of the Russian Parliament. From that position of power, Yeltsin promoted measures that precipitated the end of the Soviet Union.
Powerless and abandoned by almost everyone, Gorbachev resigned as President of the USSR on December 25, 1991. The Soviet red flag was lowered in the Moscow Kremlin, the Russian flag replaced it.
Russia took over from the USSR on the international scene: embassies, permanent post on the Security Council, and control of Soviet nuclear weapons. The end of the Cold War was announced, but the United States took the opportunity to impose its hegemony in a unipolar world.