February Revolution 1917.
By 1917, most Russians had lost faith in the leadership ability of the czarist regime. Government corruption was rampant, the Russian economy remained backward and Czar Nicholas II had repeatedly dissolved the Dumas, the Russian parliamentary groups established to placate the masses after the Revolution of 1905, each time they opposed his will. But the immediate cause of the February Revolution—the first phase of the more sweeping Russian Revolution of 1917—was Russia’s disastrous involvement in World War I. Militarily, imperial Russia was no match for industrialized Germany. Russian troops were shockingly ill-equipped for fighting, and Russian casualties were greater than those sustained by any nation in any previous war. Meanwhile, the Russian economy was hopelessly disrupted by the costly war effort, and moderates joined Russian radical elements in calling for the overthrow of the tzar.
Treaty of Brest-Litovsk
In March 3, 1918, in the city of Brest-Litovsk, located in modern-day Belarus near the Polish border, Russia signed a treaty with the Central Powers (Germany, Austria-Hungary, Ottoman Empire, Bulgaria) ending its participation in World War I (1914-18). With the November 11, 1918, armistice ending World War I and marking the Allies’ victory over Germany, the treaty was annulled. By the terms of the 1919 Treaty of Versailles, Germany was forced to give up its territorial gains from the Treaty of Brest-Litovsk.
German collapse in 1918.
Germany had effectively run out of men and supplies for home and the army. It was no longer possible to sustain the war, the injection of US manpower was far more important in making HL decide to launch the Spring Offensives than it was in deciding the war via actual combat. This extra manpower would have been decisive in 1919, so the war had to be won before that point by Germany. The Russian exit from the war did not end a need for troops in the east to secure that frontier and prop up the new governments, nor did it cure the acute problems the Germans were starting to suffer, where many divisions were viewed as only reliable to defend - the static divisions. The offensives cost Germany the best of her troops, once they had been lost only the less reliable remained.