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However, the Russian high command, in order to uphold its alliance with the French, determined to compensate for its weaknesses by mobilizing its available armies at once and sending them forward, while leaving the rest to arm and assemble later. These were the First Army under General Pavel Karlovitch Rennenkampf, stationed in [[Lithuania]] and northeastern [[Poland]], and the Second Army under General Alexander Vasilevich Samsonov, massed further south around [[Warsaw]]. These two armies, with a combined strength of around 450,000 men, were directed to advance west and north respectively upon East Prussia, converging around the city of Allenstein (modern Olsztyn, Poland). While Rennenkampf marched through the Insterburg Gap between Königsberg and the Masurian Lakes to engage Prittwitz's front, Samsonov would bypass the Lakes from the south to turn the Germans' right flank.  

However, the Russian high command, in order to uphold its alliance with the French, determined to compensate for its weaknesses by mobilizing its available armies at once and sending them forward, while leaving the rest to arm and assemble later. These were the First Army under General Pavel Karlovitch Rennenkampf, stationed in [[Lithuania]] and northeastern [[Poland]], and the Second Army under General Alexander Vasilevich Samsonov, massed further south around [[Warsaw]]. These two armies, with a combined strength of around 450,000 men, were directed to advance west and north respectively upon East Prussia, converging around the city of Allenstein (modern Olsztyn, Poland). While Rennenkampf marched through the Insterburg Gap between Königsberg and the Masurian Lakes to engage Prittwitz's front, Samsonov would bypass the Lakes from the south to turn the Germans' right flank.  

Though the Russians held a significant numerical advantage over the defenders, as well as the strategic value of an unexpectedly early offensive, they were beset by a number of disadvantages: many of the troops were poorly armed and equipped, owing to the incomplete mobilization; their artillery component was notably weak compared to the Germans'; though both Rennenkampf and Samsonov were capable officers, the latter was relatively new to his command, having previously served in the Far East and in Turkestan; and the Russians' supply lines would be inadequate once they advanced into Germany (Russia's use of a different rail gauge meant its cars could not run on German tracks). Another drawback that would prove significant was that the Russian army had neither a proper coding system in use, nor sufficient telephone or telegraph cable to run secure communication lines from front-line units to headquarters. As a result, once on German soil many military communications would be sent as "clear" (that is, uncoded) wireless messages, which the Germans frequently intercepted.  

Though the Russians held a significant numerical advantage over the defenders, as well as the strategic value of an unexpectedly early offensive, they were beset by a number of disadvantages: many of the troops were poorly armed and equipped, owing to the incomplete mobilization; their artillery component was notably weak compared to the Germans'; though both Rennenkampf and Samsonov were capable officers, the latter was relatively new to his command, having previously served in the Far East and in Turkestan; and the Russians' supply lines would be inadequate once they advanced into Germany (Russia's use of a different rail gauge meant its cars could not run on German tracks). Another drawback that would prove significant was that the Russian army had neither a proper coding system in use, nor sufficient telephone or telegraph cable to run secure communication lines from front-line units to headquarters. As a result, once on German soil many military communications would be sent as "clear" (that is, uncoded) wireless messages, which the Germans frequently intercepted.<ref>Barbara W. Tuchman, ''The Guns of August'' (1962), p. 315-20.</ref>

==Prelude==

==Prelude==

Though Russian units had made forays across the frontier as early as August 12, the main offensive opened on the 17th, when Rennenkampf's First Army crossed the border in force, headed for the Insterburg Gap; Samsonov's Second Army invaded from the south two days later. On the first day of his advance, Rennenkampf met resistance at the town of Stallupönen from a portion of the German I Corps under Lieutenant General Hermann von François, which drove the Russian 27th Division back in a chaotic retreat and took some 3,000 prisoners. This attack had been unauthorized by Prittwitz, who ordered François to fall back toward the Eighth Army's main line on the Angerapp River that night; the promising results of Stallupönen, though, and the sluggish pace of the Russian advance, convinced the general to launch a more general counterattack, in hopes of defeating Rennenkampf before Samsonov brought his forces to bear.

Though Russian units had made forays across the frontier as early as August 12, the main offensive opened on the 17th, when Rennenkampf's First Army crossed the border in force, headed for the Insterburg Gap; Samsonov's Second Army invaded from the south two days later. On the first day of his advance, Rennenkampf met resistance at the town of Stallupönen from a portion of the German I Corps under Lieutenant General Hermann von François, which drove the Russian 27th Division back in a chaotic retreat and took some 3,000 prisoners. This attack had been unauthorized by Prittwitz, who ordered François to fall back toward the Eighth Army's main line on the Angerapp River that night; the promising results of Stallupönen, though, and the sluggish pace of the Russian advance, convinced the general to launch a more general counterattack, in hopes of defeating Rennenkampf before Samsonov brought his forces to bear.

Prittwitz attacked near the town of Gumbinnen on the morning of August 20, with generally negative results. The I Corps under François enjoyed some success, assaulting the right flank of the Russian XX Corps and driving it from the field after its artillery ammunition ran out. However, the other units--the XVII Corps under Lieutenant General August von Mackensen and the I Reserve Corps under General Otto von Below--had had to hastily advance from the Angerapp and thus were not in a position to do more than launch frontal attacks, which the Russians easily repulsed. Under heavy bombardment, Mackensen's and Below's troops broke and fled the field, and by nightfall Prittwitz had ordered the whole army to pull back. Though Rennenkampf declined to pursue the retreating troops, due to concern about the state of his supply lines, reports had by now arrived of Samsonov's offensive from the south; in response, Prittwitz gave orders for Eighth Army to retreat all the way to the Vistula, which would mean abandoning most of East Prussia. That same night, he telephoned Field Marshal Helmuth von Moltke, overall commander of the German armies, to explain his intentions.  

Prittwitz attacked near the town of Gumbinnen on the morning of August 20, with generally negative results. The I Corps under François enjoyed some success, assaulting the right flank of the Russian XX Corps and driving it from the field after its artillery ammunition ran out. However, the other units--the XVII Corps under Lieutenant General August von Mackensen and the I Reserve Corps under General Otto von Below--had had to hastily advance from the Angerapp and thus were not in a position to do more than launch frontal attacks, which the Russians easily repulsed. Under heavy bombardment, Mackensen's and Below's troops broke and fled the field, and by nightfall Prittwitz had ordered the whole army to pull back. Though Rennenkampf declined to pursue the retreating troops, due to concern about the state of his supply lines, reports had by now arrived of Samsonov's offensive from the south; in response, Prittwitz gave orders for Eighth Army to retreat all the way to the Vistula, which would mean abandoning most of East Prussia. That same night, he telephoned Field Marshal Helmuth von Moltke, overall commander of the German armies, to explain his intentions.<ref>Dennis E. Showalter, ''Tannenberg: Clash of Empires, 1914'' (1991), p. 195-96.</ref> 

The planned retreat of the Eighth Army produced consternation at Moltke's headquarters, which had been under the impression that it was holding its own against the invasion. However, Prittwitz's orders were altered again the following day, August 21, when his staff convinced him a southward shift was necessary to block Samsonov's advance before the withdrawal to the Vistula could begin. Most units were accordingly reoriented away from the front of the First Army (which remained quiet), but Prittwitz rejected proposals that he counterattack Samsonov in turn, and in follow-up conversations with headquarters (Oberste Heeresleitung, or OHL), made clear his determination to take up a defensive stance behind the Vistula, and suggested reinforcements from the Western Front would be necessary even to hold that line.

The planned retreat of the Eighth Army produced consternation at Moltke's headquarters, which had been under the impression that it was holding its own against the invasion. However, Prittwitz's orders were altered again the following day, August 21, when his staff convinced him a southward shift was necessary to block Samsonov's advance before the withdrawal to the Vistula could begin. Most units were accordingly reoriented away from the front of the First Army (which remained quiet), but Prittwitz rejected proposals that he counterattack Samsonov in turn, and in follow-up conversations with headquarters (Oberste Heeresleitung, or OHL), made clear his determination to take up a defensive stance behind the Vistula, and suggested reinforcements from the Western Front would be necessary even to hold that line.

[[Category:World War I]]

[[Category:World War I]]