On 22nd June 1941 Germany attacked the Soviet Union. The attack was a surprise, and in breach of the Molotov-Ribbentrop Non-Aggression Pact of 1939 and, by that token, without a Declaration of War.
On 12th January 1943 the German 6th Army was trapped (approximately 190,000 men at that point) at Stalingrad. The Soviet Union had launched counter-offensives code named Uranus and Saturn (followed, later, by Mars).
Between 10th and 12th January 1943 the Germans at Stalingrad had lost 60,000 men: the Soviet Union had lost 26,000 there. Further north, on the Voronezh Front the Soviet Union, on 12th January 1943, committed 347,000 men to an attack on that Front against a total of Axis forces of 225,000 men. Further north, near Leningrad, the Soviet Union attacked with two armies totalling 244,000 men. The various Fronts reached from Leningrad to Stalingrad. At Stalingrad the temperature was 35 degrees below freezing.