Operation Barbarossa was the codename of the operation by Nazi Germany to invade the Soviet Union in 3 directions Leningrad,Moscow and Ukraine starting on June 22, 1941 after failing to invade Great Britain, up to December when the winter and overstretched supply lines stopped the rapid advance of the Wehrmacht. At first, the advance of the Wehrmacht was extremely fast and pushed deep into the Soviet Union; so much so that German forces were actually a little over 96.5 kilometres from Moscow. Later on, especially after the Battle of Stalingrad, German forces began to bog down and were pushed back by Soviet counterattacks.
This operation was a major military blunder by Hitler, which severely strained his troops and recources. He not only opened a two front war, but a two front war with a massive country, eventually resulting in the Allied victory. It also led to, and was the beginning of the war on the Eastern Front, which was to be host of some of the most bloody and brutal battles and worst atrocities of World War II. This front would have the most casualties by far as it is estimated that around 26,000,000 Soviets died alone as the result of the German invasion while some historians had estimated the number to be up to 47 million Soviets.
The German High Command had been planning Operation Barbarossa since 1940 or a year before the invasion actually took place and it was later decided in December to have a three pronged attack. The first prong, the largest, would head towards Moscow, while the second prong, the second largest attack, would head towards Leningrad. The final third prong, the smallest prong would head towards Kiev.