Gorlice–Tarnów offensive
The Gorlice–Tarnów offensive during World War I was initially conceived as a minor German offensive to relieve Russian pressure on the Austro-Hungarians to their south on the Eastern Front, but resulted in the Central Powers' chief offensive effort of 1915, causing the total collapse of the Russian lines and their retreat far into Russia. The continued series of actions lasted the majority of the campaigning season for 1915, starting in early May and only ending due to bad weather in October. Mackensen viewed securing a breakthrough as the first phase of an operation, which would  then lead to a Russian retreat from the Dukla Pass, and their positions north of the Vistula. Background. In the early months of war on the Eastern Front, the German Eighth Army conducted a series of almost miraculous actions against the two Russian armies facing them. After surrounding and then destroying the Russian Second Army at the Battle of Tannenberg in late August, Paul von Hindenburg and Erich Ludendorff wheeled their troops to face the Russian First Army at the First Battle of the Masurian Lakes, almost destroying them before they reached the protection of their own fortresses as they  retreated across the border. At the same time, the Austro-Hungarian Army launched a series of attacks collectively known as the Battle of Galicia that were initially successful but soon turned into a retreat that did not stop until reaching the Carpathian Mountains in late September. Over the next weeks, Russian troops continued to press forward into the Carpathian passes in the south of Galicia. In fierce winter fighting General Franz Conrad von Hötzendorf, the chief of staff of the Austro-Hungarian Army, attacked the Russians in an attempt to push them back. Both sides suffered appallingly, but the Russians  held their line. By this time half of the Austro-Hungarian Army that had entered the war were casualties. Conrad pleaded for additional German reinforcements to hold the passes. German Chief of Staff Erich von Falkenhayn refused, but in April 1915 Conrad threatened a separate peace if the Germans would not help. According to Prit Buttar, "...it did seem as if the Russian Army had been gravely weakened by the recent campaigns...both "AOK" and "OHL" knew about Russian losses and difficulties with ammunition supply. Therefore, merely reducing the pressure on the "k.u.k." Army would not be sufficient; Falkenhayn wished  to strike a blow that would permanently diminish the ability of the Russian Army to mount offensives in future..." Falkenhayn wrote Conrad on 13 April, "Your excellency knows that I do not consider advisable a repetition of the attempt to surround the Russian extreme (right) wing. It seems to me just as ill-advised to distribute any more German troops on the Carpathian front for the sole purpose of supporting it. On the other hand, I should like to submit the following plan of operations for your consideration...An army of at least eight German divisions will be got ready  with strong artillery here in the west, and entrained for Muczyn-Grybów-Bochnia, to advance from about the line Gorlice-Gromnik in the general direction of Sanok." Conrad met Falkenhayn in Berlin on 14 April, where final details of Falkenhayn's plan were agreed upon, and two days later orders were issued for the creation of the Eleventh Army. According to Buttar, the Eleventh Army would consist of the "...Guards Corps reinforced by the 119th Division, XLI Reserve Corps reinforced by 11th Bavarian Infantry Division, and X Corps. Archduke Joseph Ferdinand's Fourth Army would be subordinated to the new  German army. Eventually, 119th Infantry Division and 11th Bavarian Infantry Division were grouped together in "Korps Kneussl", and additional troops in the form of the Austro-Hungarian VI Corps were added to the Eleventh Army." Buttar goes on to state, "Impressed by the resilience of German troops on the Western Front when the French attacked in late 1914 and again in early 1915, Falkenhayn had adopted the proposal of Oberst Ernst von Wrisberg...and ordered some divisions to give up one of their four regiments and to reduce their artillery batteries from six guns to four." These forces were used  to create new divisions for the new Eleventh Army. Conrad had to bow to Falkenhayn's conditions. The joint attack would be by an Austro-German Army Group commanded by a German, whose orders from Falkenhayn would be transmitted via the Austro-Hungarian command. The Group would contain the Austro-Hungarian Fourth Army (eight infantry and one cavalry divisions) under Archduke Joseph Ferdinand, an experienced soldier. The Germans formed a new Eleventh Army made up of eight divisions, trained in assault tactics in the west. They were brought east on 500 trains. The Eleventh Army was led by the former  commander of the German Ninth Army, General August von Mackensen, with Colonel Hans von Seeckt as chief of staff. They would be opposed by the Russian Third Army with 18½ infantry and five and a half cavalry divisions, under General D. R. Radko-Dmitriev. Mackensen was provided with a strong train of heavy artillery commanded by Generalmajor Alfred Ziethen, which included the huge German and Austro-Hungarian mortars that had crushed French and Belgian fortresses. Airplanes were provided to direct artillery fire, which was especially important since ammunition was short on both sides: only 30,000 shells could be stockpiled for  the attack. Another significant plus was the German field telephone service, which advanced with the attackers, thereby enabling front-line observers to direct artillery fire. To increase their mobility on the poor roads, each German division was provided with 200 light Austro-Hungarian wagons with drivers. The German Eleventh Army was ready to start artillery operations by 1 May, with the "Korps Kneussl" deployed southwest of Gorlice, with the XLI Reserve Corps led by Hermann von François, Austro-Hungarian VI Corps led by Arthur Arz, and the Guards Corps led by Karl von Plettenberg, deployed south to north, while the  X Corps was held in reserve. Joseph Ferdinand's Fourth Army was deployed north of the Germans at Gromnik. The Russian Third Army was deployed with the X Corps led by Nikolai Protopopov, XXI Corps led by Jakov Shchkinsky, and IX Corps led by Dmitry Shcherbachev, deployed south to north. The battle. Falkenhayn moved German Supreme Headquarters, OHL (Oberste Heeresleitung), to Pless in Silesia, an hour's drive from Austrian headquarters. To prevent spying, the local inhabitants were moved out of the buildup area. In the north the German Ninth and Tenth armies made diversionary attacks that threatened Riga. On  22 April, the Germans launched the first poison gas attack near Ypres, divulging what might have been a decisive weapon merely to distract the Allies in the west. Mackensen had ten infantry and one cavalry divisions (126,000 men, 457 light guns, 159 heavy pieces, and 96 mortars) along the length of the breakthrough sector. Facing him were five Russian divisions consisting of 60,000 men but desperately short on artillery. For fire support, the Russians could only count on 141 light artillery pieces and four heavy guns. One of the four burst as soon as the battle began. The Russian supreme  commander, the Grand Duke Nicholas Nicholaevitch, learned that Germans had arrived on their flank but did not make a counter-move. On 1 May, the Central Powers’ artillery opened harassing fire, zeroing in their guns. The following morning at 0600 they began a sustained bombardment from field guns to heavy howitzers, at 0900 the mortars joined in. The huge mortar shells were especially terrifying, their blast killed men tens of meters from the explosion. The Russian fortifications were "... more ditches than trenches." According to François, "The mortars began their destructive work. The ground trembled, hell seemed to be let loose ." According to Arz, "Our tension peaked as the infantry set off from their assault positions precisely at 10 o'clock." Mackensen had already defined "lines that should be reached in a uniform and if possible simultaneous manner, without preventing the troops from collectively moving on to secure the next sector where possible." After the first day, Mackensen reported, So far, everything is proceeding well," and on 3 May he reported 12,000 prisoners had been captured. François reported, "Gorlice was almost demolished; the section of the town that had been fighting resembled a sea of ruins." Mackensen then issued orders for the  advance upon the River Wisłoka as the next objective, which constituted the Russian third and final line of defense. In the meantime, Russian reserves in the form of the III Caucasian Corps, could not provide relief for at least a day. Yet, since this corps was committed to battle piecemeal, they proved of limited value in hindering the German advance. Radko Dimitriev quickly sent two divisions to stem the Austro-German breakthrough, but they were utterly annihilated before they could even report back to headquarters. From the Russian point of view, both divisions simply disappeared from the map. On 3  May the Grand Duke Nicholas was sufficiently concerned to provide three additional divisions and to authorize a limited withdrawal. The Russian XII Corps near the Dukla Pass began its withdrawal, as did the XXIV Corps near Nowy Żmigród, signifying Russian control of the Carpathians was becoming ever more tenuous. The "Korps Emmich" 20th Infantry Division, "Korps Kneussl" was renamed after Otto von Emmich took over command, captured Nowy Żmigród. Using an intact bridge over the Wisłoka, this division was able to advance to Wietrzno. By 5 May the attackers were through the three trench lines that had opposed them, by  9 May they had reached all assigned objectives. Grand Duke Nicholas permitted a limited withdrawal, but rejected advice to construct a well fortified position far behind the frontline and then to pull back to it. At this point the Russian counterattacks grew ever more desperate, often throwing brand new recruits into battle, some armed only with grenades or wooden clubs. On 6 May, Mackensen noted, "Along the entire line from the Vistula far into the Carpathians, the enemy is retreating. Today I calculate we already have 60,000 prisoners." By 11 May, Emmich's men had reached the outskirts of Sanok , while François' reached the San River, an advance of in ten days. Mackensen's objective then became the securing of the San line, before advancing onwards to Rawa-Ruska. At this stage of the battle, according to Buttar, the Russian "X and XXIV Corps had effectively ceased to exist, while IX Corps had lost 80 per cent of its establishment strength," and the III Caucasian Corps had lost two thirds of its fighting strength. On 10 May, Vladimir Dragomirov had written Grand Duke Nikolai, "The strategic position of our forces is hopeless. In particular, I consider it my duty to  note the position of Fourth Army, which will become very dangerous if the enemy breaks through along the lower San." On 12 May, Mackensen ordered bridgeheads established at Jaroslau and Radymno. By 15 May, Jaroslau had been captured and the Germans started crossing the San there on 17 May. Radymno was captured on 24 May. Dimitriev's Third Army XXI and XII Corps were transferred under the control of Aleksei Brusilov's Eighth Army in an attempt by Grand Duke Nikolai to stem the German breakthrough. Soon after, Dimitriev was dismissed, replaced by Leonid Lesh. On 28 May, the German  XLI Corps 81st Infantry Division captured Stubno and Nakło north of the Przemyśl Fortress. On 31 May, the Germans started capturing the forts surrounding the fortress, and Brusilov ordered its abandonment. Kneussl's 11th Bavarian Infantry Division marched in unopposed on 3 June. Lesh's Third Army then retreated to the Tanew River, while Brusilov's Eighth Army retreated towards Lemberg near Gródek. Furthermore, Brusilov was ordered to relinquish his V Caucasian and XXII Corps, so that the Third Army's could form a southern flank in combination with their II Cauciasian and XXIX Corps. This Third Army southern flank , led by Vladimir Olukhov, was supposed to prevent the 'Mackensen phalanx' from gaining any additional territory. The Central Powers next objective was a continued advance towards the east from a bridgehead at Magierów, and the ultimate recapture of Lemberg, which would sever lines of communication between the Russian Northwest and Southwest Fronts. Mackensen would not only command his Eleventh Army but also the Austro-Hungarian Fourth Army, augmented with the X and XVII Corps, on his northern flank, and the Austro-Hungarian Second Army, augmented with the "Beskidenkorps", on his southern flank. The Eleventh Army Order of battle, north to  south, consisted of Karl von Behr's Corps, Emmich's X Corps, Eugen von Falkenhayn's XXII Reserve Corps, Plettenberg's Guards Corps, Arz's Austro-Hungarian VI Corps, and François' XLI Reserve Corps. The attack commenced on 13 June, and by 17 June, the Germans had pushed the front line back to positions near Gródek. The German attack commenced again on 19 June, after the previous day was spent bringing forward their heavy artillery with matching aerial reconnaissance. The Russians reverted to a headlong retreat, and on 21 June the Grand Duke Nicholas ordered abandonment of Galicia. On 22  June Mackensen's Austro-Hungarians entered Lemberg after an advance of , an average rate of per day. The Galician oil fields, crucial for the German navy, were soon back in production and 480,000 tons of badly-needed oil was captured. Aftermath. Seeckt proposed that the Eleventh Army should advance north towards Brest-Litovsk, with their flanks shielded by the rivers Vistula and Bug. Mackensen and Falkenhayn supported this strategy of attacking the Russian salient in Poland, and forcing a decisive battle. "Ober Ost", led by Paul von Hindenburg and Erich Ludendorff, would attack towards the southeast, while Mackensen turned north , and the Austro-Hungarian Second Army attacked east. The Grand Duke Nicholas issued orders that yielded to the pressure step by step, evacuating both Galicia and the Polish salient to straighten out their front line, hoping to buy the time to acquire the weapons they so desperately needed, for example 300,000 rifles. This enormous movement is known as the Great Retreat of 1915. Warsaw was evacuated and fell to the new Twelfth German Army on 5 August, and by the end of the month Poland was entirely in Austro-German hands. Order of Battle. Central Powers (arrayed north to south ): "Austro-Hungarian 4th Army" (Austro-Hungarian units unless otherwise indicated): "German 11th Army" (German units unless otherwise indicated): "Austro-Hungarian 3rd Army", Russian 3rd Army (north to south): Behind the Russian front lines: Scattered across the rear of 3rd Army: Army Reserve: