Soviet Union

Founding

The Union of Soviet Socialist Republics was created by the Declaration and Treaty of Union signed 29–30 December 1922, uniting four founding republics: the RSFSR, Ukrainian SSR, Byelorussian SSR, and Transcaucasian SFSR. The union eventually comprised 15 union republics; its capital was Moscow. It was a nominal federation but in practice a highly centralized one-party state under the Communist Party.

Lenin and the 1920s

Lenin's government fought and won the Civil War, then adopted the market-tolerant New Economic Policy (NEP) in 1921 to revive the shattered economy. After Lenin's death in January 1924, a power struggle followed a brief collective leadership (troika), from which Joseph Stalin emerged victorious by the late 1920s, sidelining and eventually eliminating Trotsky and other rivals.

Stalinism: industrialization, collectivization, terror

Stalin launched the Five-Year Plans (from 1928) driving crash industrialization, and forced collectivization of agriculture, which triggered catastrophic famine in 1932–1933 (including the Ukrainian Holodomor), killing millions; the regime concealed the toll by suppressing the 1937 census. The Great Purge (1936–1938) saw show trials and mass executions — roughly a million "enemies of the state" killed and many more sent to the Gulag — decimating the Party, military, and intelligentsia.

The Great Patriotic War (1941–1945)

After the Molotov–Ribbentrop Pact (1939), Nazi Germany launched Operation Barbarossa on 22 June 1941. Per Wikipedia, "more than 3.8 million Axis troops invaded the western Soviet Union along a 2,900-kilometer (1,800 mi) front, with the main goal of capturing territory up to a line between Arkhangelsk and Astrakhan, known as the A–A line" — the largest land invasion in history. Key turning points:

Battle of Moscow (30 September 1941 – 7 January 1942): the German advance was halted within ~20 miles of Moscow and thrown back in a counteroffensive begun 5 December 1941 — Hitler's first major land defeat.

Siege of Leningrad (8 September 1941 – 27 January 1944): lasting 872 days; History.com states the siege "killed an estimated 800,000 civilians," while the post-Soviet Russian figure puts total human losses (civilian and military) at roughly 1.5 million, with civilian losses around 1.1–1.3 million — mostly from starvation.

Battle of Stalingrad (17 July 1942 – 2 February 1943): the encirclement (Operation Uranus, planned by Zhukov and Vasilevsky) trapped the German Sixth Army; Field Marshal Paulus surrendered on 31 January and the last troops on 2 February 1943 (~91,000 prisoners). It was the decisive turning point of the war in Europe: "the Germans lost the strategic initiative and they never regained it."

Battle of Kursk (5 July – 23 August 1943): the largest tank battle in history (Prokhorovka, 12 July 1943); after defeating the German Operation Citadel, the Red Army went permanently onto the offensive.

Battle of Berlin (16 April – 2 May 1945): Marshals Zhukov, Konev, and Rokossovsky led some 2.5 million troops in the final assault; Hitler committed suicide on 30 April; the garrison surrendered on 2 May; Germany's unconditional surrender took effect 8 May (9 May Soviet time) 1945 — Victory Day.

The USSR suffered by far the heaviest losses of the war. The post-Soviet Government of Russia puts total Soviet WWII losses at 26.6 million (per the 1993 Russian Academy of Sciences study); the Krivosheev (1993 Russian Ministry of Defense) study "puts Soviet military dead and missing at 8.7 million," with the remainder civilian — about a quarter of the population killed or wounded.

Superpower and Cold War

Victory made the USSR a superpower dominating Eastern Europe behind the "Iron Curtain." Under Stalin's successors:

Nikita Khrushchev (1953/1956–1964): "de-Stalinization" (the 1956 Secret Speech), the Space Race triumphs (Sputnik, 1957; Yuri Gagarin, first human in space, 1961), the Cuban Missile Crisis (1962), and agricultural experiments; ousted in 1964.

Leonid Brezhnev (1964–1982): military parity with the US, the crushing of the Prague Spring (1968), détente, but domestic economic "stagnation."

Brief successions of Andropov (1982–1984) and Chernenko (1984–1985).

Perestroika, glasnost, and dissolution

Mikhail Gorbachev (1985–1991) launched perestroika (restructuring) and glasnost (openness), reduced Cold War tensions, and permitted the fall of the Eastern Bloc (1989). But nationalist movements surged and central control weakened. The failed August 1991 coup by hardliners fatally undermined Gorbachev and propelled Boris Yeltsin to the fore. The Belovezha Accords (8 December 1991) and Alma-Ata Protocol declared the USSR dissolved; Gorbachev resigned on 25 December and the Soviet Union formally ceased to exist on 26 December 1991. Its legal successor was the Russian Federation.