The Soviet Union (officially the Union of Soviet Socialist Republics or USSR) is a communist union made up of 16 republics which are mentioned in We Happy Few in the form of conversations by Wastrels, letters and newspapers which you can find across the districts.
In 1917, the destruction and devastation of World War I sparked the February Revolution in the Russian Empire which led to the downfall of the Tsarist government and the All-Russian Provisional Government taking its place. Just nine months later, the Provisional Government was overthrown by the Bolsheviks in the October Revolution. This sparked a long and bloody civil war in Russia between the Whites and the Reds, which ultimately ended with the victory of the Reds and in 1922, the Soviet Union was founded.
It can be assumed that history mostly followed that of our own world up until the 1930s and the outbreak of World War II in 1939. By the time when the German Empire invaded and occupied Great Britain and Wellington Wells in 1943, both powers were already at war with each other. The Soviet Union eventually sided with the USA to fight against a belligerent German Empire, ultimately winning the war.
This is evidenced by the fact that in Wellington Wells, you can find several old letters, articles, papers, and newspaper scraps asking people to give more and more resources for the "war against communism" as it is referred to in one newspaper. Based on one of these newspaper scraps, the Soviets took Berlin in 1948 and defeated the German Empire, and it can be assumed that the rest of the Axis Powers fell with them.
This did not deter the Soviet Union, however, as they invaded the rest of continental Europe, conquering France and installing a puppet regime named "the French Soviet Republic" and then continuing all the way down to Spain, defeating the Fascist government led by Francisco Franco and installing a "democratic republic" in their place. Only the UK and Ireland were left unscathed but it is unknown how well off they are.
After the end of World War II, the alliance between the Soviets and the Americans began to sour, presumably leading to some sort of Cold War, not unlike what happened in real life.
The Soviet Union is a Eurasian socialist country made up of 16 republics. It consists of modern-day Russia, Ukraine, Belarus, Kazakhstan, Uzbekistan, Kyrgyzstan, Tajikistan, Turkmenistan, Georgia, Armenia, Azerbaijan as well as the Baltic States (Estonia, Latvia, Lithuania) and Moldova.