What was the significance of the Second Battle of the Marne?

The Second Battle of the Marne marks the end of the 1918 German resurgence on the Western Front, thereafter initiative rested with the Allies.

Following the Soviet signing of the Treaty of Brest-Litovsk on March 3rd, 1918, the Germans were able to transfer over 50 divisions from the Russian Front to the Western Front. They used this bonus to launch a series of offensives aimed at shattering Allied resolve and to drive a wedge between the British and the French.

On 21 March, over three German armies attacked 5th British Army (Operation Michael) in the Somme sector and retook all the territory gained in the Arras and Somme offensives of 1916-1917, being halted outside Amiens.

On April 9th, Operation Georgette was unleashed north of the Ypres Salient, and halted three weeks later after gaining everything taken in the 3rd Ypres Offensive of 1917.

Blücher–Yorck began along the Aisne River on May 27th and ran out of steam on June 6th.

The next offensive was Gneisenau along the Matz River on June 9th and was called off two days later when checked with a French counter-offensive. The last offensive, Friedensturm, was along the Marne River east of Rheims, and began — like the others — with some German success when it began on July 15th, but a French-US counter-attack on July 18 brought it to a halt.

The German manpower advantage of March had been spent as the attrition curve had favoured the Allies. German supplies were low, and the Allies had done much to conserve their strength (particularly with premiere offensive formations like the Canadian Corps). The French counter-attack continued to August 6th, and pushed the Germans back some 25 km in 18 days.

On August 8th, at Amiens, the Canadian and Australian Corps were united to spearhead a surprise offensive that ripped the German lines open in the most devastating attack of the War. A general offensive all along the line followed once the Hindenburg line was breached in early October; and Germany’s strength rapidly ebbed away.