Editing Battle of Tannenberg (section)

==Prelude: 17–22 August==

Rennenkampf's First Army crossed the frontier on 17 August, moving westward slowly. This was sooner than the Germans anticipated, because the Russian mobilization, including the Baltic and Warsaw districts, had begun secretly on 25 July, not with the Tsar's proclamation on 30 July.<ref>McMeekin, S. (2011) ''The Russian Origins of the First World War''. Harvard University Press, p. 67.</ref>

Prittwitz attacked near [[Battle of Gumbinnen|Gumbinnen]] on 20 August, when he knew from intercepted wireless messages that Rennenkampf's infantry was resting. German [[I Corps (German Empire)|I Corps]] commanded by Gen. [[Hermann von François]] was on their left, [[XVII Corps (German Empire)|XVII Corps]] commanded by Lt. Gen. [[August von Mackensen]] in the center and [[I Reserve Corps (German Empire)|I Reserve Corps]] led by Gen. [[Otto von Below]] on the right. A night march enabled one of François’ divisions to hit the Russian XX Corps' right flank at 04:00. Rennenkampf's men rallied to stoutly resist the attack. Their artillery was devastating until they ran out of ammunition, then the Russians retired. I Corps attacks were halted at 16:00 to rest men sapped by the torrid summer heat. François was sure they could win the next day. On his left, Mackensen's XVII Corps launched a vigorous frontal attack but the Russian infantry held firm. That afternoon the Russian heavy artillery struck back—the German infantry fled in panic, their artillery limbered up and joined the stampede. Prittwitz ordered I Corps and I Reserve Corps to break off the action and retreat also.{{cn|date=August 2022}}

At noon, Prittwitz had telephoned Field Marshal [[Helmuth von Moltke the Younger]] at OHL ([[Oberste Heeresleitung]], Supreme Headquarters) to report that all was going well; that evening he telephoned again to report disaster. His problems were compounded because an intercepted wireless message disclosed that the Russian II Army included five Corps and a cavalry division, and aerial scouts saw their columns marching across the frontier.<ref>Showalter, 1991, pp. 91–94.</ref> They were opposed by a single reinforced German Corps, the XX, commanded by Lt. Gen. [[Friedrich von Scholtz]]. Their advance offered the possibility of cutting off any retreat westward while possibly encircling them between the two Russian armies. Prittwitz excitedly but inconclusively and repeatedly discussed the horrifying news with Moltke that evening on the telephone, shouting back and forth. At 20:23 Eighth Army telegraphed OHL that they would withdraw to West Prussia.<ref>Showalter, 1991, p. 195</ref>

[[File:Paul von Hindenburg (1914) von Nicola Perscheid (cropped).jpg|thumb|upright=0.9|[[Generalfeldmarschall]] [[Paul von Hindenburg]]|alt= An old photograph of a man with a moustache in military uniform.]]
By the next morning, 21 August, Eighth Army staff realized that because Samsonov's II Army was closer to the Vistula crossings than they were, they must first relocate most of their forces to join with XX Corps to block Samsonov before they could withdraw further. Now Moltke was told that they would only retreat a short distance. François protested directly to the Kaiser about his panicking superiors.<ref>Showalter, 1991, p. 196.</ref> That evening Prittwitz reported that the German 1st Cavalry Division had disappeared, only to later disclose that they had repulsed the Russian cavalry, capturing several hundred. By this point, Moltke had already decided to replace both Prittwitz and his chief of staff, Alfred von Waldersee. On the morning of 22 August their replacements, Col. Gen. [[Paul von Hindenburg]] and Maj. Gen. [[Erich Ludendorff]], were notified of their new assignments.{{sfn|Buttar|2016|pp=143–144}}

The Eighth Army issued orders to move to block Samsonov's Second Army. I Corps on the German left was closest to the railway, so it would take the long route by train to form up on the right side of XX Corps. The other two German corps would march the shorter distance to XX Corps' left. The First Cavalry Division and some older garrison troops would remain to screen Rennenkampf. On the afternoon of 22 August, the head of the Eighth Army field railways was informed by telegraph that new commanders were coming by special train. The telegram relieving their former commanders came later. I Corps was moving over more than 150&nbsp;km (93 miles) of rail, day and night, one train every 30 minutes, with 25 minutes to unload instead of the customary hour or two.<ref>Lincoln, W. B., (1986) ''Passage through Armageddon. The Russians in war & revolution 1914–1918''. Simon & Schuster, pp. 72–73.</ref>

After the battle at Gumbinnen, Rennenkampf decided to pause his First Army to take resupply and to be in good positions if the Germans attacked again. This caused them to lose contact with the German Army, which he incorrectly reported was retreating in haste to the Vistula. Both Russian armies were having serious supply problems; everything had to be carted up from the railheads because they could not use the East Prussian railway track, and many units were hampered by a lack of field bakeries, ammunition carts and the like. The Second Army also was hampered by incompetent staff work and poor communications. Poor staff work not only exacerbated supply problems but, more importantly, caused Samsonov during the fighting to lose operational control over all but the two corps in his immediate vicinity (XIII & XV Corps).<ref>{{cite book|last1=Golovine|first1=Nicholas N.|title=The Russian army in the World War|date=1931|publisher=Oxford University Press}}</ref>{{pn|date=August 2021}}

On 21 August, Samsonov's Second Army crossed the border, and quickly took several border towns.  The VI Corps took [[Ortelsburg]], while I and XV Corps advanced onto [[Soldau]] and [[Neidenburg]].  On 22 August, Samsonov ordered XV Corps to advance towards [[Olsztynek|Hohenstein]], which they did on 23 August pushing [[Friedrich von Scholtz]]'s XX Corps out of [[Lahna]].{{sfn|Buttar|2016|pp=153–159}}