Way back about this time last century, Imperial Deutschland was like totally throwing her mighty military might against France, Belgium and Great Britain's BEF in the famous Schliefflen Plan as the 1st month of combat in WWI got all crunk up.
With Imperial Russia mobilizing to attack Germany, the vaunted German General Staff took meticulously planned advantage of interior rail roads and logistics to deploy most of her troops - over 34 Corps (2 or more divisons each) - like nearly a million soldiers to overwhelm and annihilate Western Front Allied armies, accept a French surrender and cut peace deals with Great Britain and Belgium and then transport her victorious armies to the Ost Front and derail the dreaded "Russian Steamroller" as it began to finally advance on Germany.
As a scarecrow of sorts, Deutschland left her 8th Army (4 and a half Corps, a cavalry division and the Konigsberg garrison) in what was then Ost Prussia.
Imperial High Command via von Moltke (the younger) instructed 8th Army:
Defend West and East Prussia yet not get overwhelmed by superior forces or be driven into the fortress of Konigsberg. If 8th army were threatened by superior forces - it was to high tail it behind the Vistula river - leaving East Prussia to Russia until victorious truppen fresh from the West Front would reinforce, counter attack and retake it.
Russia surprised everyone with her scheme of "Forward Mobilization" and fielded two ginourmous armies happily named the 1st and 2nd armies that formed up and struck into East Prussia a month ahead of schedule.
8th army had a rowdy rebellious Corps commander who dissed orders and launched an all out attack on the Russians 1st army at the frontier. Such a move could pull 8th army corps by corps away from her prepped defensive positions into an awful killing machine leaving part of Russia's 2nd Army with a chance to cut off and destroy most - if not all of 8th army.
Another spoiling attack on 1st Army caused an additional corps to flee from the Russians, 8th army was in very real danger of being driven into Konigsberg which OHL had expressly forbidden.
"Keep 8th Army intact. Don't be driven from the Vistula, yet in case of extreme need abandon the region east of the Vistula"
8th Army commander von Prittwitz felt the extreme need was now. Ringing up OHL at Coblenz, he announced his intention to retreat to Vistula river if not behind it. He added the waters in the summer heat were at low ebb and he was doubtful if he could even hold the river without reinforcements.
High Command was shocked. Reinforcements! Where on earth could they come from save West Front - where every last battalion was engaged? Worse - Russians on Vistula river would threaten Berlin, the Austrian flank and even Vienna. Giving up the valuable dairy and grain regions of the Reich would be a tremendous morale and prestige loss.
8th Army's junior commanders argued against such a retreat. Utilizing railways on which so much brain power had been expended would allow 8th Army to regroup, refresh and mass against the nearest danger - the Russian 2nd army. A full envelopment and defeat of the Russians could be achieved and then repeat the manuever on the 1st army.
Refusing anything but withdrawal secured von Prittwitz's dismissal from 8th army.
Recalling von Hindenburg from retirement and Ludendorf from capturing the fortresses of Belgium to command 8th army they concurred with 8th Army's staff's plan to regroup and attack 2nd army.
Suddenly an extrodinary phone call all the way from OHL - pledging 3 additional corps (about 6 divisions) - came through. Ludendorf - who knew down to the last decimal the required density of manpower per kilometre - could hardly believe what he heard.
In distributing Prussian refugees across Germany, the Germans had succeeded in frightening themselves. Tearful pleas of of high born Junker ladies to the Kaiserine, vast estates left for maurading cossacks and fears of a Russian advance into the heart of the Reich pulled those corps from the victory at Namur in Belgium.
8th army argued passionately against their transfer and the 3 corps were only out of line for a week in the West.
The battle began
Two armies, now totally committed, surged and gripped and broke apart and clashed again in confused and seperate combats over a front of 40 miles for days. A regiment advanced, it's neighbor was thrown back, gaps appeared, the enemy thrust through or, unaccountably did not. Artillery roared, cavalry squadrons, infantry units, horse-drawn field gun batteries moved and floundered through villages and forrests, between lakes, across fields and roads. Three hundred thousand men flailed at each other, marched, counter marched and died as the great battle of the Eastern Front was fought out.95,000 Russians troops were captured in the action; an estimated 30,000 were killed or wounded, and of his original 150,000 total, only around 10,000 of Samsonov's men escaped. The Germans suffered fewer than 20,000 casualties and, in addition to prisoners captured over 500 guns. Sixty trains were required to transport captured equipment to Germany.
Tannenberg was a great victory for Germany, restored her faith in ultimate victory and made living legends out of von Hindenburg and Ludendorf in the 1st month of the war.
Yet withdrawing those 3 corps from West Front - even for a short time - ensured that Germany would lose the war in the coming few days...
Pic - "We had not merely to win a victory over Samsonof. We had to annihilate him."