The Union of Soviet Socialist Republics (also known as the USSR or the Soviet Union) consisted of Russia and surrounding countries that today make up Estonia, Latvia, Lithuania, Belarus, Ukraine, Moldova, Georgia, Azerbaijan, Armenia, Kazakhstan, Uzbekistan, Turkmenistan, Kyrgyzstan, and Tajikistan. The USSR was founded in 1922 and cemented itself onto the world stage in 1924, 7 years after the Russian Revolution overthrew the monarchy of the Tsar in the October Revolution. The USSR would suffer the greatest casualties during WWII, by far. By the war's end it is estimated that 20 million to 30 million people were killed with exact numbers being hard to pinpoint, due to the massive amounts of casualties. Also many of the deaths were civilians with estimates of around 2/3 being civilians whether from execution, bombing, Nazi induced starvation, etc. The USSR was a member of the "big three" of the war with their leader being Joseph Stalin. Following the events of WWII the USSR and The United States would begin the conflict known as the Cold War. It was originally a Federation of Communist states but was dissolved in late 1991 shortly after the fall of the Berlin Wall resulting in the end of the cold war with 15 new nations emerging from the chaos. The Commonwealth of Independent States was a somewhat unsuccessful effort by Russia to keep the former countries of the USSR together in an economic alliance post-dissolution.
Main page: Weapons of the USSR
The Workers' and Peasants' Red Army (RKAA) was the massive ground force of the Soviet Union. Due to the re-introduction of conscription in 1940, its numbers increased tenfold all the way up to 9 million.
Initially, it suffered from personnel purges, which all but wiped out all the combat veterans, shiftiness of doctrines, and incomplete re-armament. This, and Stalin's orders not to respond to German provocations, left it in an extremely vulnerable position, allowing the larger German invasion force to brutalise it until Operation Typhoon, at which point, bolstered by Siberian units and the civilian militia, it began to push back.
By 1945 the Red Army was a massive, hardened force that combined experienced soldiers, advanced but simple small arms, rugged and versatile tanks, and large amounts of artillery.
The People's Comissariat of the Interior, operating the secret police of USSR, also had its own military. While it was initially dedicated to border patrol, it later became associated with barrier troops and hunting down Soviet POWs (all of them considered German agents).
The Navy of the Soviet Union was impressive. But it was not a major component to World War II naval battles outside the Eastern Front. As a matter of fact, it was in many ways similar to the Kriegsmarine, with a sizeable submarine component and a slightly outdated battleship fleet while lacking any sort of aircraft carriers.
It is highly notable that only the Navy had been mobilised, contrary to Stalin's orders, before Operation Barbarossa and as a result suffered almost no casualties during the opening phase of the "Great Patriotic War".
The air force of the Soviet Union initially suffered from outdated equipment and horrendous level of pilot training, allowing Luftwaffe pilots to rack up hundreds upon hundreds of easy kills. However, it fought regardless of losses, eventually growing numerous and confident enough to turn the tables on the Germans.