
Since 1870, when Bismarck's Prussia defeated France at the Battle of Sedan, taking emperor Napoleon III prisoner and annexing Alsace and Lorraine, France had thirsted for revenge. "Think of it always, speak of it never" was the famous mantra, and for a while in the 1880s colonial rivalries almost meant Britain replaced Germany as the prime enemy. However, when war with Germany became inevitable, the French high command instigated Plan XVII - an all-out offensive to recapture the lost territories of Alsace and Lorraine. The high command considered that the french soldier's fighting spirit and elan would overcome any defences arrayed against them. They were wrong, of course, and the famous Pantalons Rouges were cut down in their thousands.

French troops en route to POW camps in Germany. This is a hand-coloured photo.
Meanwhile, in Flanders, only a combination of aircraft spotting, the heroic rearguard action of the British Expeditionary Force, German indecisiveness and luck had prevented a total allied collapse under the weight of the main German assault. General Gallieni, the commander of the Paris garrison, commandeered a fleet of taxis to take soldiers to the front lines, and at the Battle of the Marne, the Germans were stopped, then pushed back. Once the lines had solidified, the French were able to collect themselves and issue their soldiers with more up-to-date uniforms, including steel helmets (of the Adrian pattern that was used by armies across Europe well into World War II), in a pale blue shade known as Horizon Blue. It was thought that this would help camouflage the wearer against the sky as he charged across no-man's land.

French troops march to the front.
1915 saw the disastrous Dardanelles campaign, in which French naval forces played a major part, and in 1916 the French joined the British at Salonika.
However, 1916 was also the year of Verdun. The German High Command had decided to concentrate all their force against just one sector of the front, and that place was to be Verdun. The first assault began in February, and the Germans captured the key Fort Douaumont 5 days later. However, the French under Petain had decided to hold the area at all costs as a symbol of French pride. During the 7-month battle, some 20 million shells were fired and Verdun itself also ceased to exist, supplied by a single minor road (which became known as the Voie Sacree or Sacred Road and full of troops. It is estimated that some two thirds of the total front-line strength of the French Army was was rotated through Verdun at some stage. Fort Vaux, another symbol of French resistance, was finally captured by the Germans on the 9th of June, but by this stage, the German plan was simply to turn Verdun into a "meat-grinder" that would consume French men, resources and morale. Territorial gain was almost irrelevant, since the territory itself was almost worthless. The fall of Fort Vaux almost cracked the French army altogether, but the Germans were forced to withdraw men to shore up the Eastern Front and French counter-attacks re-took Vaux and Douaumont. It was heartening for the French, but all it meant was that after 7 months, more than half a million French dead and untold devastation and misery, that nothing had actually changed.
The French had "won" the battle of Verdun. The army was intact, the Germans had taken some 400,000 casualties themselves and Verdun was still in French hands. However, the horror of Verdun had taken its toll on French morale, and when, in 1917, General Nivelle (who had been commander of Verdun after Petain was promoted) ordered a massive and futile offensive against German lines which cost 118,000 lives in 4 days, the French army mutinied. The British mounted a series of diversionary attacks, and thanks to French censorship, the Germans didn't find out about the mutinies until they were over. Nivelle was sacked and replaced by Petain, who improved conditions in the trenches and promised an end to such futile attacks.
In March 1918, the Germans launched their last-ditch offensive, the Kaiserschlacht which made such inroads into French positions that German long-range artillery could shell Paris. However, the German offensive could not be maintained and the French, British and newly-arrived US forces advanced once more, in the kind of war of movement that had not been seen since 1914 on the Western Front.
France emerged from the Great War in a similar state to Great Britain: Victorious but shattered. The French Prime Minister, Georges Clemenceau, went to the peace talks at Versailles with just one objective - to wring every last penny out of Germany, both to rebuild France and to ensure that Germany could never again be a threat. The reparations imposed were unrealistic to say the least and when, in 1925, the Germans defaulted on their reparation payments, France reacted by occupying the Ruhr Valley - nub of German industry. The Saar coalfields had been awarded to France pending a local referendum, and Alsace-Lorraine had of course been restored. German and Turkish colonies were also handed out to the victorious powers as League of Nations Mandates. France acquired Syria and Lebanon, which would become independent following the Second World War.