Eastern Front (World War I)

The Eastern Front or Eastern Theater, of World War I,  was a theater of operations that encompassed at its greatest extent the entire frontier between Russia and Romania on one side and Austria-Hungary, Bulgaria, the Ottoman Empire, and Germany on the other. It ranged from the Baltic Sea in the north to the Black Sea in the south, involved most of Eastern Europe, and stretched deep into Central Europe. The term contrasts with the Western Front, which was being fought in Belgium and France. Unlike the static warfare on the Western Front, the fighting on the geographically larger Eastern Front was more dynamic, often involving the flanking and encirclement of entire formations, and resulted in over 100,000 square miles of territory becoming occupied by a foreign power.

At the start of the war Russia launched offensives against both Germany and Austria-Hungary that were meant to achieve a rapid victory. The invasion of East Prussia was completely defeated while the advance into Austria-Hungary stalled in the Carpathians, and following successful offensives by the Central Powers in 1915 its gains were reversed. Germany and Austria-Hungary defeated Russian forces in Galicia and Poland, causing Russia to abandon the Polish salient, parts of Belarus and the Baltic region, and Galicia. However, the campaigns of 1914–15 also failed to achieve Germany's objective of taking Russia out of the war, and by 1916 Germany prioritized its resources for winning in the West.

Russia went on the offensive to take pressure off of France at the Battle of Verdun: its attack near Lake Naroch in early 1916 was quickly defeated by Germany, but the Brusilov offensive that summer became the largest Entente victory in the war. Russia inflicted over one million casualties on Austria-Hungary and forced Germany to redeploy divisions from the Western Front, at the cost of its own heavy losses. In August 1916 Romania entered the war but was quickly overrun by Germany, though Russia helped prevent a total Romanian collapse. The events of the February Revolution in March [O.S. February] 1917, caused by food shortages in Russian cities, began a decline in discipline among the troops.

After the abdication of Emperor Nicholas II, the Russian Provisional Government chose to continue the war to fulfill its obligations to the Entente. In July 1917 Russia's last offensive of the war ended in failure, and in September Germany captured Riga, bringing the German Army closer to the Russian capital. This was followed by a military coup attempt that weakened the Provisional Government. The Bolsheviks overthrew the Russian Republic in the October Revolution of November [O.S. October] 1917. Despite the political instability, the majority of the Russian Army was still intact and stayed at the front line until early 1918, though the Bolsheviks began taking steps to dissolve it in December 1917 while maintaining some forces against the Central Powers as their negotiations were ongoing.

The new Soviet government established by the Bolsheviks signed the Treaty of Brest-Litovsk with the Central Powers in March 1918 after Operation Faustschlag, taking Russia out of the war; leading to a Central Powers victory. However, the Western Entente soon defeated the Central Powers, with the Treaty of Brest-Litovsk being annulled by the Armistice of 11 November 1918. Romania and the Central Powers signed a separate peace treaty on 7 May 1918, but it was canceled by Romania on 10 November 1918.

The assassination of Archduke Franz Ferdinand eventually led to Austria-Hungary's ultimatum to Serbia on 23 July 1914 with German backing, and after the Serbian response to it was rejected, Austria-Hungary declared war against Serbia on 28 July. Russia had made a decision to support Serbia and defend its interests in the Balkans before that, and on 29 July Russian emperor Nicholas II ordered a partial mobilization of the Russian Army in the military districts that bordered Austria. The following day he was convinced by his advisors to order a full mobilization to follow the military's existing mobilization schedule. Nicholas hoped Germany would see this as a regional conflict between Russia and Austria, but because their war plans depended on taking advantage of Russia's slower mobilization speed, and due to the position of both countries, they felt pressure to go to war immediately. Austria-Hungary began full mobilization on 31 July, and Germany declared war on Russia on 1 August 1914.

Russia's plan at the outbreak of war was known as Schedule 19. Approved by Emperor Nicholas II in July 1910 and amended over the next two years, the plan concentrated Russia's armies on its western border with Germany and Austria-Hungary and was initially defensive. The original Schedule 19 would have had the Russian Army mobilize its troops to locations that were a significant distance away from its western border and leave the entire Polish salient to be defended by local fortresses and the Warsaw garrison. Under this version Russia would not have taken any offensive action and would have waited to react to movements by Germany and Austria-Hungary. The defensive nature of Russian planning went back to the military reforms in the 1860s following the Crimean War and was also influenced by the limited capacity of Russia's railway network to facilitate the quick mobilization of troops, though that was gradually improving.

Russian strategy remained this way until developments that took place starting in 1911, when the Chief of Staff of the French Army visited Russia and asked the Russian high command to plan for an immediate offensive if war with Germany broke out to help prevent them from capturing Paris. This was also in accordance with Russia's 1892 promise to attack Germany if France was invaded in order to open a second front. In 1912 the Russian chief of staff, Yakov Zhilinsky, told the French that Russia would go on the offensive against Germany fifteen days after the start of its mobilization. By this point, the improvements in Russia's infrastructure made this a lot more feasible, and there was also domestic pressure for Russia to not abandon its borderlands to foreign occupation. In 1911 new proposals for modifying Schedule 19 were made by army officers, with the most notable among them being Quartermaster-General Yuri Danilov's argument for going on the offensive against both Germany and Austria-Hungary.