Why didn't France and the UK invade Germany in September 1939?

Clearly the desire to intervene was not there in the allied ranks. An early attack in the north by the British whilst the poles still held out would have spurred France on and forced the German army to falter. Would Churchill have gone for broke possibly Chamberlain never

It was a share of multiple factors.

1. France at the time had a much smaller population than Germany, from which to draft personel. It was thought that a defensive war to grind german numerical advantage was the best way to fight this war. French generals expected a repeat of world war one, and planned accordingly, with the hindsight that in the first world war, the attacks against trenches were costly and, generally, ineffective. This proved to be a wrong assumption in the long run.
2. Lack of national unity on the french part. To change the status quo you need a centralized government, else, everyone will avoid the risk changing the current thinking.
3. France lacked leadership, doctrine and means (People and Materiel) to persecute an offensive war against an armed and organized Germany.
4. Lack of interest, the stated enemy of germany was URSS, and they expected the germans and russians to kill each other on the long run.

One thing to consider is that the invasion came right after the announcement of the Soviet-German pact so the Allies were aware that it wouldn't turn in to a two front war. And in fact that the Soviets would do half the work of occupying Poland.

But primarily as other answers have touched upon France and the UK simply were not ready to launch an offensive war in September of 1939. There wasn't even a joint staff.

The French had a fortress mentality and would rather sit behind the Maginot Line than take any risky adventure.

the ineffectual Prime Minister Neville Chamberlain was paralysed by indecision and churchill did not ascend to the Prime Ministership until the invasion of France on 10th May 1940.

What you had was a vacuum of leadership. Churchill on the other hand was a military man who had been involved in the charge of the dervishes during the conflict in Sudan, where he had learned the need for decisive action.

Some great answers, starting with lack of preparedness, timidity and belief that time was on their side.

However, I am surprised no one mentioned the seasonal timing. Northern European wars typically stagnate during wintertime and offensives happen in spring, summer and fall.

By attacking on Sept 1st, Germany gave itself just enough time to complete a 4-6 week offensive before the end of campaign season. Most countries then, and now, don't have much of their force in a rapid reaction stance - it will take at least a few weeks to get a unit ready for mobile activity. But every week's delay in calling up reserves and getting ready would trim that much off the Allies' timeline, should they start attacking.

In short, the timing gave a good reason for the Allies to put things off until the next year's campaign season. Of course, we now know, with hindsight that it was the wrong thing to do, but it seemed reasonable at the time.

In addition to hoping for a negotiated solution, the Allies were unprepared for war. Germany had been rearming for years, while the Allies thought they were playing catch-up. They knew their industrial capacity was superior, though, not to mention strategic advantage on raw materials thanks to their colonies. That's why they were content to hold the line and exploit their economic advantages to rearm and build up a large modernized force that could be used in 1941 or even later.

The French did attack, starting on September 7, 1939. They even gained some ground. Attacking on September 7 in reaction to the September 1 invasion of Poland is actually remarkably quick. So the question might more appropriately be could they have attacked more strongly?

Saar Offensive (Wikipedia)

France wasn't prepared.

The French strategy in the late 30s (including the military industry which had been greatly weakened in the 30s) did'nt include attacking Germany before 1941. They could have invaded Germany (the Ruhr would have been sufficient) in 1936 (before the Anschluss and the annexion of the Czech arm industry). But it was clearly not possible in 1939 (lack of planes, lack of an effective offnsive supply chain, lack of efficient tanks, even if they had th best models at that time).

That's why they chose a defensive/attentist posture, that, strategically, was much likelier to succeed that the crazy German attack strategy that could have failed very quickly hadn't the French been so slow to react because of their rigidity, lack of efficient intelligence (or any intelligence), obsolete command structure and some unfortunate incidents (like half of the staff of the nothern French Army getting accidentally killed just before the discovery of the German attack).

Broadly speaking, there are two strategies in winning a war: "attrition" (starving the enemy into surrender) and "overwhelming" (defeating the enemy on the battlefield).

Both sides tried both strategies in the WW1, and, in the end, Entente won by attrition.

Moreover, the attempts at the battlefield victory were so costly, that the Western allies did not even consider it an option by the start of WW2.

Thus, the war plan included a rigid defense and a tight blockade, not a decisive offensive, public statements to the contrary notwithstanding.

I remember from reading Churchill's memoirs that in September 1939 it was pretty much the consensus among western generals and politicians to take the defensive strategy. Everybody remembered costly offensives of WWI and preferred to count on the Maginot Line.

Preparing for war takes place over a matter of months, if not years. This is true physically, logistically, and psychologically. Basically, the Germans were ready for war in September 1939, the Allies were not.

One advantage enjoyed by the German army was the "practice" it had obtained in the occupation of both Austria and the modern Czech Republic (Slovakia became a satellite state). There was no resistance, but an occupation is an occupation, and the German army worked out a number of logistical bugs. What's more, they got the benefit of Austrian and Czech weapons-producing capability. The Allied armies had no similar experience.

Then there was Poland, a flat land made for German tanks in ideal (non hot, non rainy) early fall weather. Imagine a Germany army on the Polish border at the race track, supplies and ammunition in place, ready to charge across the starting line at top speed as soon as the starting gun goes off, crushing everything standing its way. With some help from the Soviet Union (totally unexpected by the Allies), the Germans are across Poland in about 30 days.

On the other side, you do have Allied armies with about a 5 to 1 numerical advantage against German defenders of the Siegfried line. A fifth of those troops are in Britain, across the English Channel, and will require time to deploy. Their symbolic importance is greater than their numerical importance, because the French won't move without them.

And the Allies weren't "at the race track," but at home, "heading down to the track." They could, and did cross the German border, but the experience of World War I had taught them that even with a 5-to-1 advantage, defeating a fortified enemy would take some time, certainly more than a month. And suppose they began to get the better of the defenders of the Siegfried line...

Buoyed by their recent victory, the invaders of Poland would have hurried back across Germany and smashed the Allied attack in late fall, possibly causing as much or more damage as they did in the invasion of France. On the other hand, the French experience (pre tank) was that they could hold on for a long time in a defensive war, which is the war they adopted.

I believe the greatest mistake of the allies was made before Poland, during the Munich treaty. The Allies left Czechoslovakia to the Germans in exchange for a promise of peace which never came.

Czechoslovakia had a great defensive line, better tanks than the Germans, the same quality airplanes and totally awesome and modern artillery which the Germans totally missed. If the Allies had supported Czechoslovakia instead of throwing them to the wolves, Hitler would easily have been stopped before he started to plan to attack Poland.

Instead of that the German army nearly doubled their supplies and weapon base after the occupation of Czechoslovakia. The Germans got the most modern artillery in Europe, many tanks even better than the German ones, many airplanes, a huge load of ammunition and fuel (they had fuel for 1 week before Munich). In Czechoslovakia the Germans got also several top class factories already producing weapons. All these resources were used to conquer Poland, France and during attack the Soviet union later.

I believe this was the greatest mistake of Allies, they had chance to stop Hitler before the real war even started but they supplied them instead.

The Phoney War (Sitzkrieg, Drôle de Guerre, etc.) seems destined to remain one of the great mysteries of history. It is difficult to comprehend now, after the fact, how such an astonishing combination of missed opportunities, wishful thinking, and indecisiveness on the part of not just one, but two great powers, could have carried on for more than half a year.

The seventh episode of the 1998 documentary series Sworn to Secrecy: Secrets of War is devoted to Sitzkrieg: The Phoney War, and a good introduction. The period is also the subject of numerous books and papers— not to mention various conspiracy theories, and certain narratives of Western betrayal, especially in Poland. Full coverage is not possible in the space of an answer here, but this excerpt from William Shirer's The Rise and Fall of the Third Reich covers most the popular theories:

[D]efeatism [among] the French… the memories of how France had been bled white in the First World War… the realization by mid-September that the Polish armies were so badly defeated that the Germans would soon be able to move superior forces to the west… the fear of German superiority in arms and in the air. Indeed, the French government had insisted from the start that the British Air Force should not bomb targets in Germany for fear of reprisal on French factories.

Fundamentally the answer to the question of why France did not attack Germany in September was probably best stated by Churchill. "This battle," he wrote," had been lost some years before." … The price of those sorry Allied failures to act had now to be paid, though it seems to have beeen thought in Paris and London that payment might somehow be evaded by inaction.

I will provide a bit more detail on three factors:

1. Unpreparedness

The British and French governments held Hitler to be a bully, willing to instigate border skirmishes and bark rhetoric, but not start a full-scale war over Poland. In fact, Lord Halifax, the British Foreign Minister, believed that Hitler was about to back down; on August 31, hours before the outbreak of war, he said he had seen in Hitler “the first view of the beaten fox.” So much for that.

Both Britain and France had been re-arming in anticipation of future conflict, and the French had begun to mobilize their army as early as August 26, but the process was incomplete. French commanders reported that they would not have sufficient resources to mount an offensive until 1941–42, and even if that was an exaggeration, other papers have argued that early in the war, the British and French believed time was on their side because it would give them time to coordinate and mobilize the full strengths of their overseas empires. The calculation was not to defend Poland in the short term, but to defeat Germany in the long term.

The British forces were inadequate for mounting a full-scale offensive. The air force was concerned about bombing, because it lacked the means to stop retaliatory raids; the navy could not operate freely in the Baltic Sea; the British Expeditionary Force was quite small compared to the French army. And even the last would take took several weeks to cross the Channel, by which time Poland was already doomed.

Still, Germany had deployed most of its forces in the east, and at the Nuremburg trials, their generals testified that had France and Britain taken action early in September, the course of the war would have changed and Germany might well have been defeated.

2. Misunderstanding of modern warfare

According to William Shirer's The Rise and Fall of the Third Reich, France was bound under treaty with Poland to attack Germany within three days of the order to mobilize, and to launch a major offensive within fifteen.

Gen. Gamelin was not a timid general, but he believed that any attack required an artillery barrage, and made his offensive and defensive plans according. The French army waited in the field while fixed artillery could be brought out from storage, shipped to the front, and assembled, and wanted for planes and tanks. A direct attack on Germany in the north was not possible without violating the neutrality of Belgium and the Netherlands; in the south, the French army did invade the Saarland on September 7 to fulfill France's treaty obligation, but did not advance far into Germany, stopping short of the Siegfried Line fortifications. And shortly thereafter, the Supreme War Council decided not to proceed with invasion, and ordered the army to retreat back behind the Maginot Line.

3. Fear of wider war and hope for a peaceful settlement

And why would the Supreme War Council do that? The horror of World War I was very much in the minds of European leaders. Fear and wishful thinking led them to hold out for what now seems like a foolish amount of time.

At the outbreak of war, Germany was allied with the Soviet Union, something that became clear when Soviet forces joined the invasion two weeks later. Chamberlain and Daladier did not want to risk angering Stalin and widening the war. They might have been able to send forces to reinforce Poland from the Mediterranean, but were not yet at war with Italy, and did not want to risk provoking Mussolini.

At the same time, Hitler was intimating with diplomats that Poland would at last appease him. On September 19 he declared in a speech that he had no war aims against Britain or France, and on the 28th Germany and the Soviet Union issued a statement that the matter of Poland having been "settled" (through their conquest and partition), there was no further cause for war. And the British were in contact with disaffected Germany military officers, hoping they would influence or overthrow Hitler. It did not come to pass.