The French planned to meet and fight the Germans in Belgium, defeat them there and then continue into Germany once the best and brightest of the German army had been ground down.
The French based their plan on their experiences in WWI. In that war, not only had the Germans occupied large swaths of northern France and the coal and iron mines and related metal industry (vital to the war effort), the defensive had proven much stronger than the offensive due to the ease of moving reinforcements by rail to any threatened part of the front, while the attacked had to move by foot and horse through the former front line to exploit a breakthrough.
The German had built the Siegfried line along the border, a decent set of fortifications and defensive structures, which the French, with experience from WWI, thought too expensive to try to force their way through.
The French plan 1939 was as follows:
- The Poles are to resist as long as possible. If they are successful, the French army will launch an offensive against the Germans 14 days after the declaration of war. If not, the Poles are to retreat to the southeastern part of the country and will be supplied by the French through Romania, which was friendly towards both countries. Like the Serbian army and the Salonika bridgehead in WWI, the Polish army will keep being a threat to the Germans, and will be ready to break out once the main German force has been destroyed.
- France and Britain was negotiating with the Soviets right up to the start of the war for an alliance. Stalin strung them along and kept demanding their support for demands on Poland and Romania, which the allies did not want to grant. In reality, they had already signed the Molotov-Ribbentrop pact with its secret protocols. The Poles were in the process of retreating what was left of their army to the Romanian bridgehead when the Soviets declared war and invaded on the 17th of September 1939. This cut the Poles off from the intended bridgehead. Combined with the devastating defeat of the Bzura counterattack and the destruction in that battle of the Poznan and Pomorze armies, the Poles were pretty much done. The French then cancelled their probing attack into the Saar region and their intended offensive, as it would do them no good. They then revised their plan.
As opposed to the common misconception, the French did not rely on the Maginot line, nor did it cost them that much. The basic idea of the Maginot line was to dissuade the Germans from attacking Alsace-Lorraine and instead funneling them through Belgium – a job it did quite well. The intention was also to save manpower, as France had only about half the population of Germany – far fewer men was needed to man the fortifications than would be needed to man the border as regular infantry units. The whole line cost about 5 billion francs 1930-1939 – about 2% of the French military budget at that time.
As Poland fell, the French revised their plans. Now, they wanted to fight in Belgium. There’s several reasons for them waiting. Attacking the Siegfried line on their own (the British BEF was nowhere near ready in Autumn 1939) without the Germans distracted by the Poles or the Soviets seemed folly. Belgium had withdrawn from the allies in 1934 to declare itself neutral, and the French wanted to have the Belgian 650 000 man on its side rather than the opposite – it meant waiting on the Germans to attack Belgium. Also, by Summer 1940, the British would have their BEF fully ready, including an armored division.
So the French dug in, preparing for a long war where resources and industry would count. They ramped up tank production, ensured their supply lines to their colonies and set their society up for war production.
The new plan was:
- Wait until the British have their army in order before doing anything offensive. The Royal Navy will strangle the Germans out of vital supplies, such as food, tungsten (needed for metalworking), chrome (needed for armor), copper and oil. Trying to get Sweden to stop exporting iron ore and Finland to stop exporting nickel was also on the table. The whole affair in Norway and the threats of an expeditionary force to help Finland was more about strangling those exports to Germany than any other issue. The Germans simply got to Norway first. The Germans had been re-arming at neck breaking speed (and were close to bankruptcy several times, only bailed out by seizing the Austrian and Czechoslovak gold reserves and foreign assets) and the French were only beginning to catch up when the war started.
- If the Germans attack, it will be through Belgium. The best of the French army will then rush north together with the BEF and link up with the Belgian army. Together they will grind down the German offensive on Belgian soil, either through vicious attrition or a decisive battle. This keeps northern France, with a lot of population and industry, not even mentioning coal and iron mines, safe and free from occupation. Once the best parts of the German army have been destroyed in Belgium, the French will lead the offensive from Belgium that will flank the Siegfried line and punch into Germany’s vital Ruhr industrial and coal producing area. After defeating the German army there, France would have crippled the German ability to conduct war and thus won, with minimal casualties and devastation to France itself.
The French were reinforced in their belief that their plans were correct in the Mechelen incident in which a German liaison plane carrying the full plan for the invasion of France crashed in Belgium on the 10th of January 1940. The event caused the Germans to scrap their plan and go with von Manstein’s daring attack through the Ardennes instead.
The French considered the Ardennes impassable for large mechanized forces – their cavalry was screening the forest (5 divisions and 3 colonial cavalry brigades, however, most of their attention was to the north, towards the Belgian part of the forest) with a force of infantry behind them at Sedan (2 infantry divisions). The Germans managed, despite massive traffic jams, to get a force of 3 Panzer divisions with 771 tanks through. They brushed the cavalry aside and crashed through the French infantry. The rest is history.
The French prepared for a long war – they were right in that, it is just that it turned out to not be very long for them. For example, the French limited their air force to 1-2 combat missions per day, intending to keep them fresh and ready for continued combat for a long time, while the Germans managed to get 4-6 combat missions per plane and day, resulting in much more effective combat usage, but crews exhausted and prone to mistakes and accidents reducing their strength. By June, the Luftwaffe was almost completely worn out and needed more than a month of rest and refit before they could launch the Battle of Britain. The French also retreated parts of their air force out of range of German fighters in order to protect them from attacks on their airfields, to allow them to rest and repair planes in peace – which meant that a large part of the French air force was in the process of moving bases and unavailable at the decisive moment.
The French knew that the Germans would come through Belgium and rushed their best forces north to link up with the Belgians once they did – however, the Germans punched a large armored force through the Ardennes forest, between the Maginot line and the Franco-Belgian positions in Belgium.
The Belgians had build fortifications in eastern Belgium, but they were not coordinated with the Maginot line, and since Belgium had withdrawn from its alliance with France 1934 to become neutral, the French could not cooperate with the Belgians on defense and fortified lines.
The Belgian fortified line pretty much fell apart when one of the key elements of it, Fort Eben-Emael was seized by a handful of German paratroopers in a daring operation.
See this map: