<P> On May 15, 1917, Philippe Pétain was made Commander - in - Chief after a few weeks as Army Chief of Staff . The French Army Mutinies had begun during that period, and he restored the fighting capability of the French troops by improving front line living conditions, and conducting only limited offensives . In the Third Battle of the Aisne, fought in May 1918, French positions collapsed due to the local commander General Duchene's defiance of Pétain's recommendation of defence in depth, and Petain's pessimism saw him subordinated to the Supreme Allied Commander Ferdinand Foch . </P> <P> Germany marched through neutral Belgium as part of the Schlieffen Plan to invade France, and by August 23 had reached the French border town of Maubeuge, whose true significance lay within its forts . Maubeuge was a major railway junction and was consequently a protected city . It had 15 forts and gun batteries, totaling 435 guns, along with a permanent garrison of 35,000 troops, a number enhanced by the British Expeditionary Force . The BEF and the French Fifth Army retreated on August 23, and the town was besieged by German heavy artillery starting on August 25 . The fortress was surrendered on September 7 by General Fournier, who was later court - martialed, but exonerated, for the capitulation . </P> <P> The Battle of Guise, launched on August 29, was an attempt by the Fifth Army to capture Guise, they succeeded, but later withdrew on August 30 . This delayed the German Second Army's invasion of France, but also hurt Lanrezac's already damaged reputation . The First Battle of the Marne was fought between September 6 and September 12 . It started when retreating French forces (the Fifth and Sixth armies), stopped south of the Marne River . Victory seemed close, the First German Army was given orders to surround Paris, unaware the French government had already fled to Bordeaux . The First Battle of the Marne was a French victory, but was a bloody one: the French suffered 250,000 casualties, of which 80,000 died, with similar numbers believed for the Germans, and over 12,700 for the British . The German retreat after the First Battle of the Marne halted at the Aisne River, and the Allies soon caught up, starting the First Battle of the Aisne on September 12 . It lasted until September 28, it was indecisive, partially due to machine guns beating back infantry sent to capture enemy positions . In the Battle of Le Cateau, fought on August 26--27, the French Sixth Army prevented the British from being outflanked . The first major Allied attack against German forces since the incarnation of trench warfare on the Western Front, the First Battle of Champagne, lasting from December 20, 1914, until March 17, 1915; it was a German victory, due in part to their machine gun battalions and the well - entrenched German forces . </P> <P> The indecisive Second Battle of Ypres, from April 22--May 25, was the site of the first German chlorine gas attack and the only major German offensive on the Western Front in 1915 . Ypres was devastated after the battle . The Second Battle of Artois, from May 9--June 18, the most important part of the Allied spring offensive of 1915, was successful for the Germans, allowing them to advance rather than retreat as the Allies had planned, and Artois would not be in Allied hands again until 1917 . The Second Battle of Champagne, from September 25--November 6, was a general failure, with the French only advancing about 4 kilometres (2.5 mi), and not capturing the German's second line . France suffered over 140,000 casualties, while the Germans suffered over 80,000 . </P>

When did the french join world war 1