Battle of the Frontiers The Battle of the Frontiers was a series of battle s fought along the eastern frontier of France and in southern Belgium shortly after the outbreak of World War I . The battles represented a collision between the military strategies of the French Plan XVII and the German Schlieffen Plan . The German idea , to concentrate their forces on their right flank and wheel through Belgium to attack the French in the rear , was foiled by the movement of General Charles Lanrezac ' s Fifth Army towards the northwest to intercept them and the presence of the British Expeditionary Force ( BEF ) on his left flank . Although driven back by the weight of three German armies , the French and British rear guard actions delayed the German advance , allowing the French time to transfer their forces to the west to defend Paris , resulting in the First Battle of the Marne . Prelude . The objective of the French Plan XVII was to recapture the provinces of Alsace - Lorraine , lost in the Franco - Prussian War of 1870 , pinning the German forces on the Rhine , and to attack the German centre through the southern Ardennes . In opposition to Plan XVII , the German Schlieffen Plan anticipated the target of the French assault and determined to remain there on the defensive , holding the Alsace - Lorraine front with the minimal possible force needed to keep the bulk of the French engaged there . The main German force would be on the right wing , sweeping through neutral Belgium and then into France , descending on Paris and executing a massive envelopment manoeuvre which would trap the French army between the two German forces and lead to its rapid annihilation . The German division s would then rapidly turn about to face France ' s ally , Russia , on the Eastern Front . The German plan mistakenly assumed Britain , not formally allied with either side , would remain neutral in the conflict , and deemed the relatively small British regular army too small to be of any impact even if it did get involved . France ' s frontier defences were based on a fortified zone from Verdun to Toul and isolated fortresses such as Épinal and Belfort . To the north , France relied on the impassable terrain of the central Ardennes forest and the promise of Belgian neutrality . Thus Plan XVII called for a two - pronged offensive , north and south of the Verdun - Toul line . In the south , the First and Second Armies would attack into Lorraine . In the north the Third , Fourth and Fifth Armies would attack through the southern Ardennes towards Luxembourg . On the left flank of the Fifth Army , facing the Belgian frontier , was the BEF , which concentrated near the fortress town of Maubeuge . On the French frontier , the German forces roughly matched the French in numbers and disposition , even to the point of being divided north and south of a fortified zone between Metz and Thionville . However , in Belgium to the north , opposing the French Fifth Army and BEF , were the German First , Second and Third Armies , forming the mass of Schlieffen ' s right wing . Germany invaded Belgium on 4 August 1914 and began to besiege Liège . Despite evidence of German forces massing east of the city , General Joseph Joffre , the French Commander - in - Chief , dismissed the threat to his northern flank and persisted with Plan XVII . Indeed , Joffre welcomed the prospect of a strong German right , which would mean the German left wing against which his offensives were directed would be weaker . In 1913 , General Nöel de Castelnau , then Deputy Chief of the French General Staff , declared a German attack through Flanders was " so much the better for us " and that opinion still prevailed amongst the French high command . Even as the German armies began flowing through Belgium in mid - August , Joffre ' s Assistant Chief of Staff , General Henri Berthelot said : Alsace and Lorraine . The first French attack of the war , known as the Battle of Mulhouse was fought on 7 August . It began in the south when the French VII Corps , along with two infantry and one cavalry division , advanced into southern Alsace with the objective of seizing the towns of Mulhouse and Colmar . Mulhouse was taken without a fight on 8 August , causing enormous celebrations in France , however a German counterattack commenced on 9 August , forcing the French to begin a slow withdrawal which was finished the next day . Joffre sent a division to reinforce the attacking force , but it arrived too late to prevent the fall of Mulhouse . In response to the repulse of VII Corps and the other divisions , Joffre sent four additional divisions to form the " Army of Alsace " under the command of retired general Paul Pau . The main French offensive in the south , known as the Battle of Lorraine , began on 14 August when the First Army of General Auguste Dubail advanced on Sarrebourg while the Second Army of General de Castelnau headed towards Morhange . The French moves were welcomed by the German Sixth and Seventh Armies under the combined command of Crown Prince Rupprecht of Bavaria . Rupprecht was in charge of the German forces assigned to meet and engage the French assault in the centre until they could be enveloped by the encircling German right wing . The German rear guards , equipped with machine gun s , inflicted heavy casualties on the French infantry , still wearing their 19th - century style uniform of blue coat and red trousers . Crown Prince Rupprecht , dissatisfied with the defensive role assigned to him , petitioned his superiors to allow him a counter - offensive . This ran against the overall German plan of luring the French in , but on Rupprecht ' s repeated insistence the policy of defence in the centre was abandoned and a counter - offensive launched on 20 August against the French Second Army , which was thrown back into France . This left the French First Army ' s flank in the air , forcing it to withdraw as well . Pau ' s Army of Alsace also had to pull back , despite having retaken Mulhouse . By 22 August , the French forces were back on their start lines along the Moselle River . The Schlieffen Plan , here and elsewhere on the front , now began to unravel as Crown Prince Rupprecht launched a full - scale offensive between Toul and Épinal , starting on 23 August . However , the French forces were now occupying prepared defences and withstood the German attack , which lasted for four days . German forces locked into Rupprecht ' s offensive were therefore unavailable for the later fighting on the Marne when they may have proved decisive . Ardennes offensive . Under Plan XVII , the French offensive against the German centre through the lower Ardennes was to involve the Third , Fourth and Fifth French armies , but by the time the offensive was due to start , Plan XVII was beginning to crumble . General Charles Lanrezac , commander of the northern - most Fifth Army , alarmed by German progress in Belgium , sought permission to realign his forces away from the Ardennes and towards Belgium , shifting west into the angle between the Sambre and Meuse rivers . Joffre remained dismissive of the threat but on 12 August he allowed Lanrezac to move I Corps west to Dinant on the Meuse . When on 15 August Lanrezac reported German forces attacking Dinant , Joffre finally acquiesced , ordering the bulk of the Fifth Army to move northwest and take up positions behind the Sambre . In addition to the loss of most of the Fifth Army from the Ardennes offensive , Joffre also withdrew divisions to reinforce the Lorraine front following Rupprecht ' s counter - offensive . The Battle of the Ardennes commenced on 21 August with the attack of General Pierre Ruffey ' s Third Army towards Briey while the Fourth Army of General Fernand de Langle de Cary advanced on Neufchâteau . The German Fourth Army ( Albrecht , Duke of Württemberg ) and Fifth Army ( Crown Prince Wilhelm ) had been making a methodical advance through the Ardennes since 19 August - and on 21 August , the advance screens of the two forces collided in a confused series of skirmish es . The main forces came into contact on 22 August and in most cases the French infantry , attacking with a surplus of " élan " but a shortage of supporting artillery , were defeated by superior German tactics , particularly in their use of entrenchments , machine gun s and heavy artillery . There were isolated French successes , notably at Virton by the VI Corps but , by the evening of 23 August the French Third and Fourth Armies were in chaotic retreat , falling back on the line of the Moselle through Sedan , Stenay and Verdun . Charleroi and Mons . By 20 August , Lanrezac ' s Fifth Army had begun to concentrate on a 40 km front along the Sambre , centred on Charleroi and extending east to the Belgian fortress of Namur . On his left , the Cavalry Corps of General Sordet linked the Fifth Army with the BEF at Mons . Lanrezac ' s army of 15 divisions , weakened by the transfer of troops to Lorraine , was confronted by the 38 German divisions from the Second ( General Karl von Bülow ) and Third Armies moving southwest . Nevertheless , Joffre ordered Lanrezac to attack across the Sambre . Before he could act on the morning of 21 August , the German Second Army launched the Battle of Charleroi with assaults across the Sambre , establishing two bridgehead s which the French , lacking artillery , were unable to reduce . Bülow attacked again on 22 August with three corps against the entire Fifth Army front . Fighting continued on 23 August when the French centre around Charleroi began to fall back . Meanwhile the German Third Army had crossed the Meuse and launched a frontal attack against the French right , held by a corps commanded by General Louis Franchet d ' Esperey . The Third Army attack threatened to cut off Lanrezac ' s line of retreat but d ' Esperey ' s force stopped the German advance and delivered a successful counter - attack . However , with the evacuation of Namur and news of the French Fourth Army retreating from the Ardennes , Lanrezac ordered a withdrawal . On 22 August , BEF patrols encountered the cavalry screen of the German First Army of General Alexander von Kluck near Soignies southwest of Brussels . The British Commander - in - Chief , Field Marshal Sir John French , initially ordered an attack , believing he was confronted by an inferior force . In truth the BEF of four infantry and one cavalry division - 80 , 000 men , faced over 160 , 000 German troops — the mass of Schlieffen ' s right wing . Fortunately for the British , French chose to take up defensive positions along the Mons - Condé Canal , on the night of 22 August . The Battle of Mons began on 23 August when Von Kluck , required to maintain contact with Bülow ' s Second Army on his left , launched a frontal attack on the British line with the main weight falling on the British II Corps of General Horace Smith - Dorrien . The massed rifle fire of the professional British soldiers — the Old Contemptibles — inflicted heavy casualties on the Germans who attacked " en masse " over terrain devoid of cover . The British held up the German advance until the evening when , now aware of the size of the opposing force , and with Sordet ' s cavalry in retreat leaving the right flank exposed , they began retiring to a second defensive line . For the moment , Von Kluck made no attempt at pursuit , allowing the BEF to disengage on 24 August . Aftermath . With the failure of Plan XVII , the French and British armies began the Great Retreat to the Marne River , pivoting on the fortress of Verdun . Fearing annihilation , French favoured withdrawing the BEF behind the Seine to refit , but the British Secretary of State for War , Lord Kitchener , despite having never favoured the BEF ' s role alongside the French army , now insisted they remain in contact . The Schlieffen Plan , while still on course , was beginning to come to pieces under the tentative control of Moltke . The opening of the Russia n offensive in East Prussia and the unforeseen attack in Lorraine had drained the German right wing that was descending on Paris . Von Kluck ' s decision to take his First Army east of the French capital exposed his right flank which the Allies exploited in the First Battle of the Marne in early September , halting the German advance . For the French , their hope of a swift , decisive victory had evaporated and they now sought to avoid defeat . For Joffre , the reason for the failure of Plan XVII lay with his subordinates , claiming defeat stemmed from " grave shortcomings on the part of commanders . " Any generals , of corps or division , who in Joffre ' s opinion had failed to perform , were ruthlessly sacked . Despite the wisdom of his decision to retreat instead of counter - attacking on 23 August , General Lanrezac was sent into retirement and replaced by Franchet d ' Esperey , starting his rapid rise through the ranks . By the end of August , the French Army had suffered 75 , 000 dead of which 27 , 000 were killed on 22 August alone , making it a day to rival the first day on the Somme for bloodshed . Total French casualties for the first month of the war were 260 , 000 of which 140 , 000 were sustained during the climactic final four days of the battle of the Frontiers .