Why didn`t France & Great Britain invade Germany in 1939/40?

When the Second World War started on 1 September 1939 Germany (and the Soviet Union) invaded Poland. And although both Great Britain and France had guaranteed Poland to do everything they could to preserve the Polish state they did nothing.

But why didn`t Great Britain & France help Poland in 1939? Why didn`t both powers invade the weakly defended western part of Germany?

In 1939 neither Great Britain nor France felt prepared for war. Both estimated that it would take them until 1941 to gather enough forces. So they decided to postpone all major offenses as long as possible which between September 3, 1939, and May 10 1940 resulted in the „Phoney war“. There was only a minor french push in 1939 into the Saar Basin after the Germans had retreated from it.

Let`s find out more…

To understand why France and Britain didn`t attack Germany in 1939 it is important to also keep the events prior to the start of WW II in mind. Because of that, I will start with a short overview.

After that, we will go into why France and Britain didn`t attack Germany in 1939.

Events leading up to the beginning of World War II

The French and British declaration of war on Germany was a direct result of Germanys’ invasion of Poland.

But the alliances that forced Great Britain and France to declare war on Germany were made in the months before September 3, 1939.

The British assurance to Poland

Rumors that Germany was heavily interested in Poland circulated as early as late March of 1939. That forced Neville Chamberlain to take action.

The British Prime minister Neville Chamberlain insured the Polish government on March 31, 1939, that both France and Great Britain would do everything they could to protect the territorial integrity of the Polish state.

Now that might sound pretty good. Mighty Great Britain reinsured Poland that it would have its back. There was just one problem.

The British government had not thought about the actual implications such an alliance could have.

The whole action was barely meant as a signal to Hitler. Secretly Great Britain didn`t really oppose Hitlers’ wish to reintegrate the city of Danzig, which had been taken from Germany after World War I, into the Third Reich.

Another problem was the practicability of such an alliance between Poland and Great Britain.

Both Neville Chamberlain and the Polish marshal Pilsudski knew that (in case Hitler would act) Great Britain was currently in no situation to send financial or military aid to Poland.

There were several (surprising) reasons why Great Britain was so weak during early World War II. Please check out my article here for more information.

The official alliance between Poland, France, and Great Britain came into effect in late August of 1939 but had already shown its effect in April when the intention of such an alliance was made public.

Hitler’s reaction to the Polish-French-British alliance

As a reaction to the alliance between Poland, France, and Britain in 1939 Hitler canceled both the Non-Aggression Pact with Poland (from 1934) and the Anglo-German Naval Agreement (from 1935).

Then Hitler broke off all diplomatic relationships with Poland and Great Britain and waited.

He was probably hoping that Great Britain would once again give in to his wishes. And it’s actually hard to blame him for that hope since the same strategy had already worked in the years before. A good example is the German occupation of Czechoslovakia in 1938/1939.

Meanwhile, Great Britain tried its best to convince the Soviet Union to also promise to protect the Polish sovereignty.

The problem was that Great Britain wanted a pact that would basically burden the Soviet Union with the actual fighting while Britain would only have loose responsibilities. That obviously didn`t sit well with Stalin.

Neville Chamberlain was no longer interested in including the Soviet Union in the alliance that would ensure Polish sovereignty. He just wanted to use the negotiations to scare off Hitler.

That kind of back and forth encouraged Hitler to drive his plans to invade Poland forwards.

But he needed a partner for that…preferably a partner that shared Hitlers’ interest in Poland.

The Molotov-Ribbentrop pact

And that partner was the Soviet Union.

On August 23 1939 the german minister of Foreign affairs Joachim von Ribbentrop and his Soviet counterpart Vyacheslav Molotov signed the Molotov-Ribbentrop-pact.

The Molotov-Ribbentrop pact was a non-aggression pact with a secret additional protocol in which Poland was split up between Germany and the Soviet Union and the Baltics were added to the Soviet sphere of interest.

It is noteworthy that in August of 1939 both Hitler and Stalin estimated that Great Britain & France would once again keep away from interfering with the German expansion and give up Poland without declaring war.

The start of World War II

On August 31 1939 Adolf Hitler gave his generals the order to start the attack on Poland. By the way, prior to that date, the German military had done its best to stage the attack as an act of self-defense.

More on the little-known actions that were taken by the Germans to stage the attack on Poland as an act of self-defense in my article here.

On the morning of September 1, 1939, the Polish government called for Great Britains’ aid. But they were ignored.

On the evening of September 1, 1939, Hitler received a British demand to withdraw all troops from Poland. That demand was explicitly not written as an ultimatum.

And at 9 am on September 3, 1939, the British government requested Hitler to stop all aggression within 2 hours or Great Britain would declare war.

Since Hitler didn`t react at all Great Britain entered the war on September 3, 1939, at 11 am. France reluctantly followed at 5 pm.

And that brings us to the question of why Great Britain and France didn`t immediately attack Germanys’ western borders although they had the numerical advantage.

Why didn`t France & Great Britain invade Germany in 1939/40?

In September of 1939 France actually had the numeric advantage in the west. A total of 85 french divisions stood against 34 german divisions since the bulk of the German forces were still in Poland.

So the natural question is why France (and Great Britain) did not strike against the Third Reich when they had the advantage.

Since it took quite some time until British divisions landed in France I will focus on the French troops for now.

The first month of war between Germany and France

The first two British divisions landed in France in early October 1939, one entire month after Great Britain had declared war on Germany.

More on why the British military was so weak during early WW II in my article here.

So during September of 1939, France was basically on its own.

The French (and British) hesitance to go on the offensive against Germany in 1939 can be explained by the bad experiences that the generals had made with the offensives during the early days of WW I.

During the first weeks of World War I France fought without really being well prepared. Just one example: The french infantry of 1914 was still wearing red trousers.

Now at some point in history colorful uniforms served an important point, more on that here in my article. But with technological improvements, they just became unnecessary and even dangerous to wear.

The French and British generals felt that in order to be able to win a war against Germany they would need more time to accumulate more soldiers and war materials.

Because of that France and Great Britain estimated that they wouldn`t be able to start their offenses against Hitler Germany until 1941. In 1939 their goal was to gain as much time as possible to assemble more soldiers and war materials.

In the meanwhile both France and Britain planned to wage a defensive war that would give both countries the time to profit off their superior economic and financial position.

In addition to that, the plan was to weaken the already fragile German economy by starting a sea blockade against Germany.

The Royal Navy during the first months of WW II

The Royal Navy was tasked with ensuring the sea blockade. The Royal Navy hadn`t suffered under the restrictions that had weakened the British army before WW II, more on that in my article here.

But during the early months of WW II, it suffered heavy losses.

Especially since its job was not only to enforce the sea blockade, secure allied convoys, and transport troops to France but also to hunt down German submarines and „Panzerschiffe“ (=armored ships) like the cruiser Admiral Graf Spee.

The focus on hunting german submarines and Panzerschiffe can be attributed to the First Lord of the Admiralty, Winston Churchill.

But while the Royal Navy was operating not only in the English Channel but also in the Atlantic (the Admiral Graf Spee was fought by the Royal Navy off the shore of Uruguay) the land-based forces didn`t have a lot to do.

The Phoney War – A defensive stalemate and a small french offensive

France and Britain were relying on the so-called Maginot-line, a strong defensive structure that secured the eastern border of France, to buy them time. And since most of the German divisions were still in Poland the German troops in the west also stayed behind the Siegfried line, the German equivalent of the Maginot line.

Now during September 1939, French troops were able to advance into the territory of the Saar Basin, which was basically the no-mans-land between the Maginot Line and the Siegfried Line.

But the French army could only advance because the german troops had pulled back from the territory of the Saar Basin.

The French advance into the territory of the Saar basin in September of 1939 stopped after only around 5 miles because the french general Maurice Gamelin did not want to engage the german troops yet.

During the first month of WW II, both French and German soldiers pretty much stayed within their defensive positions.

The German propaganda tried to convince the French soldiers that peace would be in their best interest and that they should end their alliance with their old enemy Great Britain.

The Polish Capitulation – effects on the western front

It wasn`t until October 6 1939 that things started moving at least on a diplomatic level.

After Poland had capitulated Hitler offered France and Great Britain peace under his conditions.

In early October 1939, Hitler offered France & Great Britain peace if they would accept the fact that Poland was spit up between Germany and the Soviet Union.

More on how Poland was split and the actions Germany took to stage acts of polish aggression as a justification of the war in my article here.

That was unacceptable for both France and Britain. Hitler had crossed a red line when he ordered the invasion of Poland and neither France nor Britain were willing to dump Poland.

And now during early October the first two divisions of the British Expeditionary Force had landed in France and were on their march to secure the French-Belgian Border. Another two British divisions would follow in the second half of October.

Plans to end the „Phoney War“

Theoretically, France and Great Britain were in a better position than Germany. They had more soldiers, a stronger economy, the Maginot Line, a sea blockade hurting the German economy, and the longer they could delay a confrontation the more weapons and soldiers (also soldiers from the colonies) would arrive in France.

Hitler knew that. And he also knew that he would have to act quickly.

Hitler originally planned to start his attack on France on November 12, 1939. But due to bad weather, the operation had to be postponed 20 times until May 10, 1940.

The time between October of 1939 and May 10 1940 was used by all sides to increase the output of the arms industry.

That time benefited France and Great Britain while it became clear that due to the sea blockade and the fragility of the german industry Germany was not able to wage a long war.

To secure the iron ore of Norway and to get ahead of the British troops (who were also preparing an invasion of neutral Norway and had already begun to mine their waters) Hitler started the invasion of Denmark and Norway on April 9, 1940.

And one month later, on May 10 1940 the Wehrmacht began their attack on France by bypassing the Maginot-Line through the Ardennes.

But both the battle for France and the invasion of Denmark & Norway are stories for another time.

I hope you enjoyed our trip into the time of the Phoney War.

Take care of yourself because you deserve it. You really do.

Until next time

Yours truly

Luke Reitzer


P. Clarke: Hope and Glory, Britain 1900-1990 (London 1996).

A. Marwick: A History of the Modern British Isles, 1914-1999 Circumstances, Events and Outcomes (Oxford/Malden 2000).

P. Dewey: War and Progress, Britain 1914-1945 (London/New York 1997).