At the outbreak of War, Maurice Gamelin, who was the "généralssime", general in command of the armies of France suggested offensive action towards Germany. The German forces were still mostly busy in Poland and there was thus a window for attack.

The french government lead by Albert Lebrun and successively Edouard Daladier and Paul Reynaud (both prime ministers) proned a more defensive campaign approach to this war.

First of all to set things in context, France essentially lost an entire generation during world war I and was, just like many European states, still recovering from the war. In prevision of future conflicts, France decided to build the famous Maginot line, with the hope of establishing a nigh impregnable line of defense. The Maginot line extended from the borders with Switzerland to Belgium. The reason it was not extended to the Channel is due to France's decision not to disrespect Belgium's neutrality policy prior to the start of the conflict. Trusting in the belief that France and Great Britain would be capable to hold off the German armies on the East part of France thanks to the Maginot line, the Allied armies would be able to mobilize troops and equipment rapidly to defend the Northern border of France, where the Maginot line wasn't in effect.

Basically, the allied strategy was born in the facts of the past world war, where it was a war of positions (trenches, constant artillery fire on the enemy positions, charges, slow and bloody war). However the battle of France proved to be a war of movements, where armies use mechnaized power to outflank or cut in half enemy armies. In this case, the german Blitzkrieg (Guerre éclaire in french, litterally "lighting war", as in speed war) shattered the Alied armies hopes to "hold the line".

In retrospect, France had developped ruthless tanks (Char B1, Somua tank), even outgunning their opponent's Panzer divisions. Great Britain had made vast improvements to Air power, with the famous Spitfire (introduced in the Royal Air force just before the war). With the vast amount of Infantry troops and Navies, the Allied armies looked in good shape and were functionning in a more autonomous way.

Germany on the offhand, sort of introduced the concept of Combined arms, using their air force, infantry and mechanized units jointly. The Air force would strike defensive targets, protecting the fast advance of the german tanks followed by many infantry divisions (establishing logistics allong the way, fuel depots, repair stations...). Overall, France and Great Britain didn't adapt to the technological advancements in time to stop the german war machine.