From 1936 onwards Hitler had steadily expanded Germany's borders to take over neighbouring regions and countries with German speaking populations.
In August 1939 Hitler signed the Nazi-Soviet non-aggression pact with Josef Stalin, leader of the Soviet Union. This meant the two countries would agree not to attack each other. They also agreed, in secret, to divide Poland between them and for the next 18 months the non-aggression pact held between the two countries.
On September 1st German troops invaded Poland from the west and on September 17th the forces of the Soviet Union invaded from the East. Poland was soon divided between these two giants.
By 1941 Hitler was in control of most of western Europe. He now felt sufficiently confident enough to pursue his policy of eastwards expansion to create "Lebensraum" or "Livingspace" by invading the Soviet Union and building a German empire in Eastern Europe.
Hitler's armies attacked the Soviet Union in Operation Barbarossa from 22nd June 1941. Stalin was taken by surprise as Hitler's forces swept into the USSR and devastated the Soviet forces. The plan was to divide the Soviet Red Army into two groups and encircle them before they could withdraw deeper into the Soviet Union.
German advances were swift during the summer and The Red Army lost huge numbers of tanks and aircraft and suffered around 4 million casualties.
By September 1941 German forces had surrounded Leningrad and were not far from the Soviet capital of Moscow.
In the south, over half a million Soviet troops were trapped near Kiev and when Hitler's troops advanced on Moscow in October Stalin seriously considered surrendering.
Hitler was delighted with his victories, however, the German forces were hampered by poor supply networks and had underestimated the determination of Soviet troops who managed to withdraw into the heartland of Russia. Moscow and Leningrad were stubbornly defended and as winter set in The Red Army began a series of counter-attacks.
As the extreme cold of the Russian winter hampered the German advance Hitler sacked his three army group commanders and assumed direct command of the campaign. His troops were ordered to dig in and form defensive "hedgehog" formations. Meanwhile superhuman efforts by Soviet troops and civilian workers meant that by the spring of 1942 the Red Army was ready to fight.