In March 1918, Bolshevik Russia signed the Treaty of Brest-Litovsk with the Central Powers ending the Great War on the Eastern Front. This freed up Germany for Operation Michael and a last great Spring Offensive on the Western Front.
The German Empire began their Kaiserschlacht on 21 March 1918, with the first of four offensive operations. The global conflict that had begun in late summer 1914 had turned into a war of attrition and Germany was having an increasingly difficult time with food and supplies. After defeating Russia, the Austria-Hungary Empire was occupied on the Italian Front and in occupation of Romania, leaving Germany to fight the Western Front in Flanders and France. With the American entry in 1917, it became clear to the German High Command that decisive action was necessary before U.S. troops became a decisive factor. And almost a year after President Wilson’s declaration of War, the U.S. Army was just getting mobilized in France, the main theater of the Western Front.
The German army had also adopted training and tactics that had proven successful against the Russian army on the Eastern Front. For example, new elite Stormtrooper (Stoßtruppen) assault troop units specialized in small, fast-moving infiltration tactics, by-passing heavily defended infantry positions to cut communications and supply lines. British and French troops, by contrast, had been bogged down in trench warfare along the 300 mile long Hindenburg Line since the winter of 1916, with little opportunity for innovation.
The 21st of March was a Thursday in 1918. At 4:30 in the morning, the Germans opened an artillery bombardment on British positions at Saint-Quentin, in the Aisne area of northern France. This area, along with Somme and Pas-de-Calais areas in the modern Hauts-de-France region, had been devastated in the Battle of the Somme in 1916. The bombardment spread with mortars, mustard gas, chlorine gas, tear gas and smoke canisters filling the foggy day along a 40 mile front. The objective was to isolate the British from the French and force a British withdrawal back across the Channel. On the first day, the Germans gained ground. Friday and Saturday (22-23 March) were also foggy, and British troops fell back in fighting retreats to maintain communications. The lines were badly fragmented by Sunday the 24th and Monday the 25th, and some British units along the Somme retreated with the French army. The French were increasingly concerned with defending routes to the capital in Paris, which was in range of German long-range artillery.
On Tuesday the 26th of March, leading French and British leaders (including Winston Churchill) met at the Doullens Conference in an attempt to form a more unified command. French General Ferdinand Foch was tapped, officially given the title of Commander-in-Chief later, on 3 April, at Beauvais, including American forces and later Italian forces. Unified Allied command proved essential to defeating the Central Powers through the rest of 1918.
The Germans continued to advance over 26th-28th, advancing about 40 miles into France at one point, with a final assault coming Saturday the 30th on the French south of the Somme and on the British near Amiens, with fighting continuing through the 4th and 5th of April. While the German army had captured ground and gained morale, they had not achieved their strategic objectives. Both sides had about 250,000 casualties apiece, with little difference in situation. If anything, by being forced to accept unified command, the Allies came out stronger for their losses.
Operation Michael was followed by Georgette (Battle of Lys) in April, Blücher-Yorck (3rd Aisne) in late May, and Gneisenau in June, generally with slight German gains in territory with casualties comparable on both sides. However, by July the advantage swung to the Allies. The German Army had lost about 1,000,000 fighting men in the first 6 months of 1918 and faced ever increasing challenges with supplies. With the first major American action begun, the Allies recovered from the Spring Offensives and went on the counter-offensive that ultimately won World War I.