Situation in the East
As dictated by the Schlieffen Plan, only General Maximilian von Prittwitz's Eighth Army was allocated for the defense of East Prussia as it was expected that it would take the Russians several weeks to mobilize and transport their forces to the front (Map). While this was largely true, two-fifths of Russia's peacetime army was located around Warsaw in Russian Poland, making it immediately available for action. While the bulk of this strength was to be directed south against Austria-Hungary, who were only fighting a largely one-front war, the First and Second Armies were deployed north to invade East Prussia.
Crossing the frontier on August 15, General Paul von Rennenkampf's First Army moved west with the goal of taking Konigsberg and driving into Germany. To the south, General Alexander Samsonov's Second Army trailed behind, not reaching the border until August 20. This separation was enhanced by a personal dislike between the two commanders as well as a geographic barrier consisting of a chain of lakes which forced the armies to operate independently. After Russian victories at Stallupönen and Gumbinnen, a panicked Prittwitz ordered the abandonment of East Prussia and a retreat to the Vistula River. Stunned by this, Moltke sacked the Eighth Army commander and dispatched General Paul von Hindenburg to take command. To aid Hindenburg, the gifted General Erich Ludendorff was assigned as chief of staff.
Before his replacement arrived, Prittwitz, correctly believing that the heavy losses sustained at Gumbinnen had temporarily halted Rennenkampf, began shifting forces south to block Samsonov. Arriving August 23, this move was endorsed by Hindenburg and Ludendorff. Three days later, the two learned that Rennenkampf was preparing to lay siege to Konigsberg and would be unable to support Samsonov. Moving to the attack, Hindenburg drew Samsonov in as he sent Eighth Army's troops in a bold double envelopment. On August 29, the arms of the German maneuver connected, surrounding the Russians. Trapped, over 92,000 Russians surrendered effectively destroying the Second Army. Rather than report the defeat, Samsonov committed suicide.
Battle of the Masurian Lakes
With the defeat at Tannenberg, Rennenkampf was ordered to switch to the defensive and await the arrival of the Tenth Army which was forming to the south. The southern threat eliminated, Hindenburg shifted the Eight Army north and began attacking the First Army. In a series of battles beginning September 7, the Germans repeatedly attempted to encircle Rennenkampf's men, but were unable to as the Russian general conducted a fighting retreat back into Russia. On September 25, having reorganized and been reinforced by the Tenth Army, he launched a counter-offensive which drove the Germans back to the lines they occupied at the start of the campaign.
Invasion of Serbia
As the war began, Count Conrad von Hötzendorf, the Austrian Chief of Staff, vacillated over his nation's priorities. While Russia posed the greater threat, national hatred of Serbia for years of irritation and the assassination of Archduke Franz Ferdinand led him to commit the bulk of Austria-Hungary's strength to attacking their small neighbor to the south. It was Conrad's belief that Serbia could be quickly overrun so that all of Austria-Hungary's forces could be directed towards Russia.
Attacking Serbia from the west through Bosnia, the Austrians encountered Vojvoda (Field Marshal) Radomir Putnik's army along the River Vardar. Over the next several days, General Oskar Potiorek's Austrian troops were repulsed at the Battles of Cer and Drina. Attacking into Bosnia on September 6, the Serbs advanced towards Sarajevo. These gains were temporary as Potiorek launched a counter-offensive on November 6 and culminating with the capture of Belgrade on December 2. Sensing that the Austrians had become overextended, Putnik attacked the next day and drove Potiorek out of Serbia and captured 76,000 enemy soldiers.
The Battles for Galicia
To the north, Russia and Austria-Hungary moved to contact along the border in Galicia. A 300-mile long front, Austria-Hungary's main line of defense was along the Carpathian Mountains and was anchored by the modernized fortresses at Lemberg (Lvov) and Przemysl. For the attack, the Russians deployed the Third, Fourth, Fifth, and Eighth Armies of General Nikolai Ivanov's South-Western Front. Due to Austrian confusion over their war priorities, they were slower to concentrate and were outnumbered by the enemy.
On this front, Conrad planned to strengthen his left with the goal of encircling the Russian flank on the plains south of Warsaw. The Russians intended a similar encircling plan in western Galicia. Attacking at Krasnik on August 23, the Austrians met with success and by September 2 had also won a victory at Komarov (Map). In eastern Galicia, the Austrian Third Army, tasked with defending the area, elected to go on the offensive. Encountering the General Nikolai Ruzsky's Russian Third Army, it was badly mauled at Gnita Lipa. As the commanders shifted their focus to eastern Galicia, the Russians won a series of victories which shattered Conrad's forces in the area. Retreating to the River Dunajec, the Austrians lost Lemberg and Przemysl was besieged (Map).
Battles for Warsaw
With the Austrian's situation collapsing, they called upon the Germans for aid. To relieve pressure on the Galician front, Hindenburg, now the overall German commander in the east, pushed the newly formed Ninth Army forward against Warsaw. Reaching the Vistula River on October 9, he was halted by Ruzsky, now leading the Russian Northwest Front, and compelled to fall back (Map). The Russians next planned an offensive into Silesia, but were blocked when Hindenburg attempted another double envelopment. The resulting Battle of Lodz (November 11-23) saw the German operation fail and the Russians almost win a victory (Map).