On September 1, 1939, Hitler’s Germany invaded Poland, triggering the outbreak of the Second World War. Two days later, on September 3, Britain and France declared war on Germany, invoking their pledge to guarantee Polish independence.
Yet, when on September 17, 1939, the Soviet Union under Joseph Stalin also invaded Poland from the east, no declaration of war followed from London or Paris.
Why?
How could Britain and France — bound by solemn promises to defend Poland’s territorial integrity — react militarily to the German invasion but ignore the Soviet one?
The answer lies in a web of strategic, political, and diplomatic calculations. Behind that decision were the principles of realpolitik, fear of communism, the balance of power, and the geopolitical vision that would shape not only the war but also the entire Cold War order that followed.
1. The Molotov–Ribbentrop Pact: Prelude to Disaster
To understand Britain’s actions in September 1939, we must begin with the German–Soviet Non-Aggression Pact, signed on August 23, 1939, by Joachim von Ribbentrop and Vyacheslav Molotov.
The pact, publicly presented as a simple non-aggression treaty, contained a secret protocol dividing Eastern Europe into German and Soviet spheres of influence.
Poland was to be partitioned along the Narew–Vistula–San line: the west to Germany, the east to the Soviet Union.
In other words, Berlin and Moscow had already agreed on Poland’s destruction before the invasion even began.
Caught between two totalitarian powers, Poland’s fate was sealed.
2. Britain’s Guarantee to Poland: A Political Gesture More Than a Military One
After Hitler’s occupation of Czechoslovakia in March 1939, Britain finally abandoned its policy of appeasement.
On March 31, 1939, Prime Minister Neville Chamberlain announced to the House of Commons that Britain would assist Poland “in the event of any action which clearly threatened its independence.”
However, this commitment was not unconditional.
The Anglo-Polish Agreement, formalized on August 25, 1939, made it clear that British assistance would be triggered only by a German attack.
A Soviet attack was not explicitly included.
Behind this legal ambiguity were two main factors:
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Britain feared Germany more than the USSR. Hitler’s expansionism was seen as the immediate threat to Europe’s balance of power.
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London still hoped for an understanding with Moscow against Hitler. Declaring war on the USSR would have made any future alliance impossible.
3. The German Invasion and Britain’s Declaration of War
When German troops crossed the Polish border on September 1, 1939, Britain and France sent ultimatums demanding immediate withdrawal.
When Hitler ignored them, both countries declared war on September 3.
And yet, despite this formal declaration, the Western Allies did almost nothing militarily to help Poland.
The so-called “Phoney War” on the Western Front followed — months of inactivity while Poland was crushed in a matter of weeks.
4. September 17: The Soviet Invasion
While German forces advanced from the west, on September 17, the Red Army invaded from the east, claiming that the Polish state had “ceased to exist” and that Moscow’s troops were merely protecting Ukrainian and Belarusian minorities.
London and Paris were stunned.
The Polish ambassador to London, Edward Raczyński, demanded that Britain declare war on the Soviet Union as well, since the guarantee covered Poland’s independence and territorial integrity.
But Chamberlain and Foreign Secretary Lord Halifax refused.
They sent a mild diplomatic protest to Moscow — and nothing more.
No declaration of war followed.
5. Why Britain Did Not Declare War on the Soviet Union
Several intertwined motives explain Britain’s decision.
a) Legal and Formal Considerations
The Anglo-Polish Agreement defined a German attack as the trigger for British intervention.
From a legalistic perspective, the government argued that the clause did not automatically apply to the Soviet Union.
This narrow interpretation conveniently allowed London to avoid a second, unwinnable war.
b) Military Impossibility
In September 1939, Britain lacked the military capability to fight two major continental powers simultaneously.
Its army was still mobilizing, and the French were entrenched defensively behind the Maginot Line.
To declare war on both Germany and the USSR would have been strategic suicide.
c) Distrust of Communism — But a Desire to Keep Options Open
While Britain viewed the Soviet Union and communism with deep suspicion, its leaders also believed that the German–Soviet pact was temporary.
They expected Berlin and Moscow to clash eventually.
Therefore, it was wiser to keep communication channels open with Stalin, who might one day become a useful ally against Hitler.
A declaration of war would have foreclosed that option entirely.
d) Public Opinion and Political Realism
British public opinion was outraged by the German invasion but did not perceive the USSR as the main enemy.
For most people, the immediate aggressor and destabilizer of Europe was Hitler, not Stalin.
Chamberlain’s government, already under fire for appeasement at Munich, was reluctant to expand the conflict or appear reckless.
6. Western Ambiguity Toward the USSR (1939–1941)
Britain’s decision inaugurated a period of strategic ambiguity toward the Soviet Union.
London formally condemned Soviet actions — the annexation of eastern Poland, the Baltic States, and the attack on Finland — but refrained from direct confrontation.
When Stalin invaded Finland in November 1939 (the Winter War), Britain and France supported the Finns diplomatically and even planned limited military aid, but they stopped short of open war.
Everything changed in June 1941, when Germany launched Operation Barbarossa — its invasion of the Soviet Union.
From that moment, the USSR became Britain’s de facto ally.
Churchill, despite his lifelong anti-communism, captured the pragmatic shift in one memorable phrase:
“If Hitler invaded Hell, I would at least make a favorable reference to the Devil in the House of Commons.”
7. Britain’s Strategic Vision: Containing Germany, Not Saving Poland
A key reality often overlooked is that Britain did not go to war to save Poland, but to contain Germany.
The guarantee to Warsaw was a political instrument to deter Hitler, not a moral crusade for Eastern Europe’s freedom.
Once Poland fell within weeks, Britain’s focus shifted entirely to preparing for a long confrontation with Germany.
Poland became a cause, a symbol, but not a military objective.
The alliance with the Polish government-in-exile in London was genuine, yet it never altered Britain’s fundamental goal: defeating Hitler first, whatever the cost to Eastern Europe.
8. Britain and the Polish Question During the War
After the double invasion, the Polish government-in-exile relocated first to France and later to London, where it was officially recognized by Britain.
Polish forces fought heroically alongside the Allies — in the Battle of Britain, in North Africa, and in Italy at Monte Cassino.
Yet, Britain’s attitude toward the Soviet Union remained ambivalent.
When news emerged of the Katyn massacre (1940), in which thousands of Polish officers were executed by the NKVD, the British government downplayed the issue to avoid diplomatic tensions with Moscow.
Likewise, when Stalin installed a communist government in Poland in 1944–45, Churchill and Roosevelt, at Yalta, accepted Soviet dominance in Eastern Europe as a fait accompli.
For the Poles, it was a bitter betrayal — a tragic repetition of 1939.
9. The Long-Term Consequences
Britain’s refusal to declare war on the USSR had far-reaching consequences.
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It allowed the Soviet Union to consolidate control over Eastern Europe, paving the way for the Iron Curtain and the Cold War.
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It revealed that British foreign policy was driven by strategy, not morality.
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It left Poland doubly betrayed: first by being abandoned in 1939, then by being sacrificed again at Yalta in 1945.
Ultimately, the decision underscored a timeless truth of international politics: great powers act not for ideals but for the preservation of their own interests and balance of power.
Conclusion
Britain’s decision not to declare war on the Soviet Union in 1939 was not hypocrisy but strategic calculation.
The British government knew it could not fight both totalitarian powers at once.
Germany represented the greater and more immediate threat.
Moreover, London foresaw that, sooner or later, Hitler and Stalin would clash — and that Britain would need the USSR as an ally to survive.
The choice was coherent with the logic of realpolitik, yet it sealed the fate of Poland and Eastern Europe.
Behind the noble rhetoric of “defending freedom” lay a cold understanding: in the game of great powers, small nations are expendable.
When in 1945 Poland emerged “liberated” yet under Soviet control, history had come full circle — back to the fateful September of 1939, when Britain decided to fight one tyranny while tolerating another.