|1914 Invasion of East Prussia|
|Part of Eastern Front of World War I|
German prisoners after the Battle of Tannenberg
|Commanders and leaders|
| Nikolai Dukhonin|| Paul von Hindenburg|
| First Army|| Eighth Army|
|Casualties and losses|
|56,671 killed or wounded||104,593 killed, wounded, or captured|
|Russian invasion of East Prussia|
|Stallupönen • Gumbinnen • Tannenberg|
The Russian invasion of East Prussia (also called the East Prussian Operation or East Prussian Offensive) occurred during the First World War, lasting from August to September 1914. As well as being the natural course for the Russians to take upon the declaration of war with Germany, it was also an attempt to focus German military eyes on the Eastern Front, as opposed to the Western Front, where France was increasingly under the strain of her own German invasion. It resulted in a capture of East Prussia by Russia from the Germans, and began the Vistula-Oder Offensive. The invasion was a devastating blow for German morale and the war effort.
The Germans initially planned to have only the 8th army to act, as they expected that the Russians would be slow to mobilise, leaving Germany to beat France in a few weeks thereby allowing the victorious, battle-hardened German troops to transfer along Germany's superior transport network to fight the Russians on the Eastern Front. This was the basis of the Schlieffen Plan.
However, quite quickly, Russia was able to mobilize an invasion into East Prussia. Any invasion of Prussia was an important blow to German morale as well as her general strategic situation, due to Prussia (including East Prussia) being the historical heart of the German Reich (Empire). The German deployment on the outbreak of the war left only the 10 divisions of the German Eighth Army under General Maximilian von Prittwitz in East Prussia whereas the Russians had been able to mobilize the First Army, under General Nikolai Dukhonin and the Second Army, under General Alexander Samsonov. They entered East Prussia on 7–9 August.
The Battle of Stallupönen, fought between Russian and German armies on 17 August 1914, was the opening battle of World War I on the Eastern Front. It was a minor Russian success, but did little to upset the Germans' plans. This was followed by the Battle of Gumbinnen, where a larger German force was defeated and given considerable casualties on 20 August. The Eighth Army staff began to panic and ordered a counteroffensive near Tannenberg. Paul von Hindenburg replaced Prittwitz, who was sacked. At Tannenberg, the Russian Second Army inflicted heavy losses against the Eighth Army, forcing its remnants to retreat behind the Vistula River.
The East Prussian offensive was a success, leaving East Prussia in Russian hands and opening up a route further into Germany. This, combined with the Russian victory in the Galicia offensive, greatly boosted Russian morale. Meanwhile, the Germans panicked, afraid that Berlin itself may be in danger.