From Catastrophe 1914: Europe Goes to War, by Max Hastings (Knopf, 2013), Kindle Loc. 4060-4087:

On that same dreadful 22nd [of August 1914], the French Fourth Army advanced up a forest road through the Ardennes which led through the village of Bellefontaine. One regiment, led by Charles Mangin, headed onwards until, as they approached Tertigny, the Germans opened fire from neighbouring woodland. Bitter fighting followed; Mangin led a bayonet charge, while street fighting developed in Bellefontaine, which came under heavy shellfire. That evening, French survivors retired to the edge of the forest, having lost eight company commanders and more than a third of the regiment. France had always planned to exploit its colonial mercenaries to make good its shortfall of white manpower. Mangin wrote in a deplorable book he published in 1910, La Force noir: ‘In future battles these primitives, for whom life counts so little and whose young blood flows so ardently, as if eager to be shed, will certainly display the old “French fury” and will reinvigorate it if necessary.’ Now that war had come, Moroccans, Senegalese and Algerians were indeed hurled foremost into its flames. By 1918, France’s black soldiers had suffered a death rate three times higher than that of their white comrades, because they were so often selected for suicidal tasks.

One of the first of these fell to the 3rd Colonial Infantry Division. On 22 August its units advanced in column through the village of Rossignol, and thence up a narrow road into the Forêt d’Anlier. The French had made no attempt to reconnoitre: horse, foot and guns merely marched into the midst of the woodland, led by the Chasseurs d’Afrique, nicknamed ‘les marsouins’ – ‘porpoises’, because of an old naval connection. Germans already deployed among the trees waited patiently until the entire division was committed, then unleashed a torment of fire which, within the space of a few minutes, shattered the formation. Trapped on the narrow road, horses, men, carts and guns milled in chaos, until the fortunate contrived to surrender. The division lost 228 officers and 10,272 other ranks, including 3,800 men taken prisoner; two generals were killed, one wounded and captured. Indeed, almost all the French commanders perished: among the divisional artillery, only a single officer survived.

This massacre was achieved solely by rifle and machine-gun fire, for artillery was useless in the dense woodland. After the war, a memorial was erected by the father of one of the dead, Lt. Paul Feunette. The grieving parent never forgave himself, because he had responded to his son’s pre-war sowing of wild oats by insisting that he should join the Chasseurs d’Afrique ‘to sort him out’. After the French retreated, the Germans conducted another orgy of violence against civilians, murdering 122 people in Rossignol on 26 August.

The fighting on this one day, the 22nd, cost the French army 27,000 men killed, in addition to wounded and missing in proportion. This was a much larger loss than the British suffered on 1 July 1916, the first day of the Battle of the Somme, which is often wrongly cited as the First World War’s high-blood-mark. Other advances upon Longwy and Neufchâteau were shattered in similar fashion to those further south. The casualties in August 1914 were not merely statistically more terrible, but dealt the French army a blow from which it never fully recovered – it is remarkable that it recovered at all. Fourth Army commander Langle de Cary observed laconically to Joffre: ‘On the whole, results hardly satisfactory.’ More than a few senior officers lost offspring: both Foch’s only son and his son-in-law perished. The C-in-C urged a renewal of the assault, but Langle ignored him and withdrew.