In Eastern Prussia, the northern Russian armies were crushed by German forces at the Battles of Tannenberg and the Masurian Lakes in late August and early September. Tannenberg, in particular, became an early symbol of Great War carnage: almost 70,000 Russian soldiers were killed and wounded during the five days of fighting, with a further 100,000 taken prisoner.

Further south, in the Habsburg province of Galicia, Russian forces fared much better, winning an important victory at the Battle of Lemberg (23 August-1 September 1914) and forcing Germany to send reinforcements to support its stumbling Austrian ally.

By late 1914, the war on the Western Front had settled into a grinding pattern of trench warfare. In the East, where fighting took place on a much longer front line, a war of movement continued throughout 1915. On 22 March, the Russians captured the Habsburg garrison of Przemysl, resulting in the surrender of 120,000 soldiers and forcing the Germans to bail out the Habsburg army again.

German troops under General Mackensen launched a counter-offensive at the nearby Galician towns of Gorlice and Tarnow in May. This local attack triggered the collapse of the entire southern flank of the Russian line. Przemysl was retaken in early June, by which time hundreds of thousands of Russian troops had been killed, wounded or captured. Further north, German troops also forced back their Russian counterparts, seizing Warsaw in early August, Brest-Litovsk on 25 August and Vilna on 19 September.

The heavy losses sustained in this 'great retreat' destroyed the pre-war Russian army, forcing military commanders to rely more heavily on inexperienced and uncommitted conscripts. On 22 August,Tsar Nicholas II, a man with little military experience and few leadership skills, appointed himself the new supreme commander of the Russian army in place of his uncle, the Grand Duke Nicholas.

After the abdication of Nicholas II in March 1917, the new Provisional Government pledged to continue the Russian war effort. But the Russian army was no longer a viable fighting force. Two million men deserted in March and April. Bolshevik agitators - including Lenin, who had returned to Russia from exile on 3 April - spread effective anti-war propaganda. A major new Russian offensive in Galicia in July 1917 failed, and by September, the northern Russian army had collapsed.

After the Bolshevik revolution in November 1917, Russia's continued participation in the First World War was doomed. An armistice signed by Germany and Soviet Russia on 15 December 1917 ended hostilities on the Eastern Front. In March 1918, the Treaty of Brest-Litovsk - a 'shameful peace' in the eyes of many Russian patriots - confirmed the extent of the German victory in the East.