During September 1939 both Germany and the Soviet Union invaded Poland. Both Britain and France had pledged to defend Poland. So why didn’t the allies declare war on the Soviet Union as well as Germany?
Before we look at the reasons it would be useful to give some background and context to this question.
Anglo-Polish Military Alliance
In response to Nazi Germany’s occupation of Czechoslovakia, Britain and France pledged their support to Poland guaranteeing its independence in the face of Nazi aggression
As the world waited with baited breath – expecting a new World War to break out anytime, two implacable ideological enemies surprised everyone by signing a Nonaggression Pact. Nazi Germany and the Soviet Union signed what was to be known as the Molotov–Ribbentrop Pact.
The clauses of the pact provided a written guarantee of peace towards each other, declaring that neither government would ally itself to, or aid an enemy of the other party. What wasn’t made public were the secret protocols that defined the borders of Soviet and German “spheres of influence”.
The Pact was a tacit agreement to divide up Poland between Germany and the Soviet Union.
Germany Invades Poland
Hitler attacked Poland, he effectively did so without fear of Soviet intervention.
Soviet Union Invades Poland
Soviet leader Joseph Stalin ordered the Soviet invasion of Poland from the east. Some of these areas were to remain part of the USSR at the end of World War II.
So, the scene is now set to answer the question of why Britain and France didn’t also declare war on the Soviet Union when the Red Army also marched into Poland in 1939.
The reason why Britain didn’t declare war on the Soviet Union is an intriguing one. Unknown to the general public there was a ‘secret protocol’ to the 1939 Anglo-Polish treaty that specifically limited the British obligation to protect Poland to ‘aggression’ from Germany.
the Poles had “understood” that “the agreement should only cover the case of aggression by Germany.”
When people questioned why Britain did nothing when the Red Army moved on Poland, the British government considered revealing the existence of the secret part of the agreement. However, they decided not to, Sir Alexander Cadogan of the Foreign Office explaining privately that to do so “would only provoke curiosity about the existence of similar secret protocols attached to other treaties…”
An answer given in the House of Commons in October 1939 revealed only that the Poles had “understood” that “the agreement should only cover the case of aggression by Germany.”
Behind the scenes, the British felt there was a clear balance to be struck between ‘morality’ and traditional, old-fashioned, national self interest
From the outset, Soviet aggression was treated differently to German aggression. On a practical level this was because the British had already shown that they could not defend Poland against one aggressor, let alone two. But it was also because the mandarins in the Foreign Office considered the eastern borders of Poland somewhat ‘fluid’ – after all, they had only been fixed in the treaty that ended the Polish-Soviet War less than 20 years earlier.
Sir William Seeds, British ambassador to Moscow, wrote in a secret telegram on 18 September 1939:
“I do not myself see what advantage war with the Soviet Union would be to us…” and that “our war aims are not incompatible with reasonable settlement [in Poland] on ethnographic and cultural lines.”
Behind the scenes, the British felt there was a clear balance to be struck between ‘morality’ and traditional, old-fashioned, national self interest. Yet, in the popular consciousness, this war is still considered almost a crusade against all evil.