On September 1, 1939, Germany launched an invasion of Poland, prompting Britain and France to declare war on Germany shortly thereafter. However, when the Soviet Union invaded Poland from the east on September 17, Britain did not extend its declaration of war to include the Soviet Union. This decision is rooted in the nature of the agreements and diplomatic calculations at the time.

In August 1939, the United Kingdom and Poland signed the Agreement of Mutual Assistance, which committed both nations to mutual military support in the event of aggression by a European country. A secret protocol within this agreement specified that Britain was obligated to intervene immediately if Germany attacked Poland. However, it only required consultation and coordination should another European power, such as the Soviet Union, carry out an attack.

As a result, the UK was under no formal obligation to declare war on the Soviet Union following its incursion into eastern Poland. This legal distinction was a significant factor influencing British policy.

Beyond the legal framework, British leaders held strategic and diplomatic reservations about confronting the USSR. Key figures, including Foreign Secretary Lord Halifax and Winston Churchill, then serving as First Lord of the Admiralty, opposed direct military engagement with the Soviet Union. They feared that antagonizing the USSR might close the door to future cooperation in the war against Germany.

At that time, the durability of the Molotov-Ribbentrop Pact—a non-aggression treaty signed between the Soviet Union and Germany in August 1939—was uncertain. British policymakers recognized the possibility that the pact could collapse, at which point the USSR might become a crucial ally against Nazi Germany.

This combination of limited legal obligation under the British-Polish treaty and the strategic preference to avoid conflict with the Soviet Union led Britain to refrain from declaring war on Moscow following its invasion of Poland. It reflected a broader calculation aimed at preserving diplomatic flexibility in a rapidly evolving and complex international situation.