Operation Barbarossa (German: Unternehmen Barbarossa; Russian: Операция Барбаросса, romanized: Operatsiya Barbarossa) was the invasion of the Soviet Union by Nazi Germany and many of its Axis allies, starting on Sunday, 22 June 1941, during the Second World War. It was the largest land offensive in human history, with over 10 million combatants taking part. The operation, code-named after Frederick Barbarossa ("red beard"), a 12th-century Holy Roman emperor and German king, put into action Nazi Germany's ideological goal of conquering the western Soviet Union to repopulate it with Germans. The German Generalplan Ost aimed to use some of the conquered people as forced labour for the Axis war effort while acquiring the oil reserves of the Caucasus as well as the agricultural resources of various Soviet territories including Ukraine and Byelorussia. Their ultimate goal was to create more Lebensraum (living space) for Germany, and the eventual extermination of the indigenous Slavic peoples by mass deportation to Siberia, Germanisation, enslavement, and genocide.

In the two years leading up to the invasion, Nazi Germany and the Soviet Union signed political and economic pacts for strategic purposes. Following the Soviet occupation of Bessarabia and Northern Bukovina, the German High Command began planning an invasion of the Soviet Union in July 1940 (under the codename Operation Otto). Over the course of the operation, over 3.8 million personnel of the Axis powers—the largest invasion force in the history of warfare—invaded the western Soviet Union along a 2,900-kilometer (1,800 mi) front, with 600,000 motor vehicles and over 600,000 horses for non-combat operations. The offensive marked a massive escalation of World War II, both geographically and with the Anglo-Soviet Agreement, which brought the USSR into the Allied coalition.

The operation opened up the Eastern Front, in which more forces were committed than in any other theater of war in human history. The area saw some of history’s largest battles, most horrific atrocities, and highest casualties (for Soviet and Axis forces alike), all of which influenced the course of World War II and the subsequent history of the 20th century. The German armies eventually captured some five million Soviet Red Army troops. The Nazis deliberately starved to death or otherwise killed 3.3 million Soviet prisoners of war, and millions of civilians, as the "Hunger Plan" worked to solve German food shortages and exterminate the Slavic population through starvation. Mass shootings and gassing operations, carried out by the Nazis or willing collaborators, murdered over a million Soviet Jews as part of the Holocaust.

The failure of Operation Barbarossa reversed the fortunes of Nazi Germany. Operationally, German forces achieved significant victories and occupied some of the most important economic areas of the Soviet Union (mainly in Ukraine) and inflicted, as well as sustained, heavy casualties. Despite these early successes, the German offensive stalled in the Battle of Moscow at the end of 1941, and the subsequent Soviet winter counteroffensive pushed the Germans about 250 km (160 mi) back. German high command anticipated a quick collapse of Soviet resistance as in Poland, analogous to the reaction Russia had during WWI. However no such collapse occurred, instead the Red Army absorbed the German Wehrmacht's strongest blows and bogged it down in a war of attrition for which the Germans were unprepared. The Wehrmacht's diminished forces could no longer attack along the entire Eastern Front, and subsequent operations to retake the initiative and drive deep into Soviet territory—such as Case Blue in 1942 and Operation Citadel in 1943—eventually failed, which resulted in the Wehrmacht's defeat.