By 1939 France had already recuperated the lost territories of Alsace and Lorraine and had no desire to increase its territory. The population had suffered greatly during the previous war, the industrial north was in ruins and would need further years to recuperate.
Military thinking was entirely given over to defence.
France would stand behind its Maginot Line (completed in 1935) whilst at the same time being prepared to outwit a second outing of the Schlieffen plan should the Germans attempt (as they had done in 1914) to swing through Belgium.
The French government refused to take up the idea of having a professional army (an idea proposed by the young Colonel de Gaulle in 1934) and remained faithful to conscription.
The army possessed 32 active divisions reinforced by the Category B Reserve Divisions made up of men who had seen little service (or more to the point, would have forgotten most of what they had been trained). If the country had to go to war then the objective was to gain time and allow the Category B units to gain in experience.
In adopting a completely defensive role the French seriously underestimated how much war had moved on since 1918. The horse had mostly (but not entirely) been replaced by motorised infantry units who would be able to cover the ground much faster than von Kluck’s foot sloggers in 1914.
In his book de Gaulle pressed the idea that tanks needed to be deployed in concentrated units and not scattered here and there in support of the infantry. It was a lesson that the higher commands simply refused to accept.
Co-ordination between infantry and armour was almost non-existent. The renowned military historian Jean-Louis Crémieux-Brilhac recalled that when he left the St Cyr academy on the 20th January 1940 the only tanks he had seen were at the 14th July Bastille Day parades.
In the event of the Germans invading Belgium/Netherlands the French Northern Group of Armies (including the BEF) would advance to the Dyle River running almost north south from Antwerpen. There they would make their stand.
This line provided good coverage for all the ports, gave France a large buffer space and offered an excellent jumping off line for an eventual counter attack towards the Rhineland.
An addition to this Dyle Plan was later made by the French Commander in Chief, Général Gamelin. Instead of keeping his 7e Armée in reserve near Reims he moved it northwards towards the coast and Dunkerque. In the event of war they would move to the Dutch town of Breda.
It could be argued that this new Dyle/Breda Plan was somewhat contrary to the rest of French strategy — a holding battle. By thinking of an immediate counter-attack Gamelin was overlooking the fact that the French Army was geared for defence.
If the French were looking for a holding war the Germans were the exact opposite, realising that they had to go for the jugular in one all encompassing lunge.
For Germany the war would be controlled foremost by its economy. Hitler could not afford a protracted engagement and as such counted greatly on the air superiority of the Luftwaffe and the speed of his mechanised units.
One of the German War Plans was in effect a quasi-rerun of the 1914 campaign plan but designed to swing through Belgium and meet the Allies in a frontal attack (as opposed to sweeping around behind them and crushing them in a vice).
Numerous arguments were put forward and ultimately Hitler accepted an outline from Lieutenant-General Erich von Manstein. A holding battle would be fought in Belgium to draw the French and British northwards to defend (by then) neutral Belgium, and the Netherlands.
The French feared a second Schlieffen plan and the Germans would tempt them with exactly that. The French and British would rush northwards to protect Belgium — which was exactly what the Allies were intending to do.
As the French moved their best units into Belgium (The 7e Armée) a second thrust would be made through the Ardennes, onto Sedan and then westwards towards the English Channel cutting the Allied forces off from their supply lines and chain of command.
The fortress based Maginot Line stopped short of Sedan as the French High Command insisted that the Ardennes were impenetrable en masse. The road system was poor and anything that did emerge at the French border would be nipped off as it did so.
There was also the delicate political matter that whilst it was being constructed Belgium was allied with France (It later redeclared its neutrality). It would have been insensitive to build fortresses facing an ally.
And so, German doctrine was for a rapid victory with much depending on the Allies falling into the trap. In a 1918 like scenario Hitler knew that if he was going to win a war against the Entente of France and the UK it had to be done soon and regardless of problems. Germany’s economy was not strong and building up her military strength had cost the civil population dear.
They had to strike like lightening.