Below: Serbian victims of the reprisals by Austria after the assassination of Franz Ferdinand,

In August 1914, on the Eastern Front of World War I, Russian forces invaded German East Prussia. The Russian army planned to attack from two directions: the First Army under General Rennenkampf and the Second Army under General Samsonov, aiming to encircle the German forces.

The German Eighth Army was initially commanded by General Prittwitz, but later command was taken over by Hindenburg and Ludendorff. The Russian army advanced through the terrain of East Prussia, which was full of swamps and lakes, but supplies were insufficient, and the soldiers were exhausted and half starved. Even so, Samsonov's army fought fiercely against the German Francois Corps for four days. 

However, on August 27, the German von Bellow Corps attacked the northern flank of Samsonov's army, and the Mackensen Corps followed suit, forming an encirclement.

Realizing that his army was being surrounded, Samsonov decided to retreat, but the German army pursued him fiercely. The particularly aggressive commander Francois continued to advance and on August 30th, he launched a surprise attack on two Russian corps while they were marching. As a result, about 60,000 soldiers were taken prisoner, and the total number of prisoners eventually reached nearly 100,000. Despairing over the crushing defeat, General Samsonov committed suicide on the battlefield.

Meanwhile, the First Army, led by Rennekamp, was unable to provide sufficient support to Samsonov's army during the crisis at Tannenberg. Isolated, Rennekamp positioned his troops along the defensive line around Lake Masurian and prepared for a German counterattack. The German Eighth Army's advance was delayed due to fatigue from a month of fighting, and the Russian army fought while preventing an encirclement on its right flank. Ultimately, the Russian army retreated towards the Niemen River, but managed an orderly withdrawal, avoiding complete annihilation. 

The subsequent Battle of Masurian Lakes resulted in a German victory, effectively driving the Russian army out of East Prussia. While not as dramatic as Tannenberg, this victory liberated East Prussia and allowed the numerically inferior German forces to seize the initiative on the Eastern Front. Moreover, this victory elevated Hindenburg to the status of a national hero.

For Russia, however, it was a catastrophe. The Second Army was effectively destroyed, and its commander was killed in action. The defeat at Tannenberg had a profound impact on the Russian army.