Along with fighting the main Allies in Western Europe, the Axis was also fighting the Soviet Union to the east. After being betrayed by Germany, the USSR quickly joined forces with the Allies and sought revenge in June 1941.
The Eastern Front was fought in northern, eastern, and central Europe.
It was a terrible conflict that was larger and costlier than the Western Front. The cruelties inflicted on soldiers and civilians by both sides resulted in 30 million deaths out of the 70 million total of World War II.
The Eastern Front was the largest and deadliest struggle in human history. Both Russians and Germans were terrorized, tortured, and murdered up and down the Eastern Theatre until the war ended.