On 22 June 1941, Nazi Germany launched a massive surprise invasion of the Soviet Union, an operation codenamed Barbarossa. This pivotal moment in the Second World War marked the opening of the vast Eastern Front, a theatre of conflict that would eventually become the largest and bloodiest in history. By turning its military might against the Soviet Union, Germany fundamentally transformed the strategic landscape of the war, forcing the Third Reich to commit the bulk of its resources to a brutal land campaign in the east.

For Britain, which had been standing alone against Germany since the fall of France in 1940, the invasion brought a significant shift in the global balance of power. The sudden entry of the Soviet Union into the conflict meant that Adolf Hitler was now engaged in a major two-front war. This development provided the British government with a vital, albeit unlikely, ally, as the pressure of sustaining the war effort in the west was now shared with the Soviet forces resisting the German advance.

The invasion had several immediate consequences for the progression of the war:

• It forced Germany to divide its military strength between the Eastern and Western Fronts.
• It established an essential, if uneasy, alliance between the United Kingdom and the Soviet Union.
• It signalled the end of the non-aggression pact previously held between Germany and the Soviet Union.
• It drew the Soviet Union into the wider global conflict, significantly altering the Allied strategy.

The opening of the Eastern Front necessitated a new level of cooperation between the British and Soviet governments, leading to the formalisation of the Anglo-Soviet Agreement in July 1941. This diplomatic shift was crucial for Britain, as it offered the prospect of wearing down German military capacity through a protracted and exhausting struggle in the east. The invasion remained a defining feature of the remainder of the war, dictating the movement of troops, the allocation of supplies, and the eventual path toward the defeat of Nazi Germany.