Central powers offensive at 1915 on the Eastern front
The Gorlice breakthrough occurred in the May 1–10, 1915 as part of the Gorlice–Tarnów Offensive during World War I. The German 11th Army led by General August von Mackensen, with the support on the flanks by the 3rd and 4th Austro-Hungarian Armies, defeated the 3rd Russian Army. For the first time in the history of the First World War a heavily fortified and long-term defensive position was broken through during the Gorliсе offensive.
Background.
From August 1914 until early spring 1915, the German Empire prioritized war on the Western front to their Eastern front. This allowed Russian troops to steadily advance through Galicia, taking the Austrian territory, including the Hungarian fortress of Przemysl in March and seizing its weapons stores. Progress was not easy, though and the Russian Imperial Army suffered heavy casualties. No matter how hard they were hit, they managed to recover so quickly that Prince von Bulow reportedly once said, "fighting the Russians [was] like pounding a pillow.".
As a result of the Russian advance into Austria and chatter that Italy (which was neutral) was preparing to enter the war on the side of the Allies, the situation was seen as dire for the Austrian-Hungarian empire. The Central Powers reorganized their armies and Field Marshall von Hindenburg who believed that Russia "could be defeated in a single campaign, provided enough troops were at hand" gave Mackensen command of the newly created German Eleventh Army with Colonel Hans von Seeckt as his Chief of Staff. 
Offensive operations of the Central Powers on the Russian front until May 1915 were carried out without numerical superiority, often with equality of forces. The key to the success of the Gorlice breakthrough was built on the creation of an overwhelming superiority in forces and means. "A quick and stunning strike in Galicia, carried out by large forces in order to achieve a decisive change in the situation in the eastern theater of operations" – this is how von Falkenhayn described the idea of the offensive on April 18, 1915. Having no advantage in manpower and machine guns in general, the Central Powers achieved an advantage in the front of the strongest of the Russian armies of the Southwestern Front and overwhelming superiority in the main attack zone (2.5 times in manpower and machine guns, almost 6 times in artillery).
Another factor of superiority and success was to be the artillery support of the operation. By the beginning of the fighting, each light battery had 1,200 shells, 15-cm howitzer batteries had 600, and batteries of 21-cm mortars had 500. 
On April 27, Mackensen outlined the requirements for the operation to subordinate commanders: "The attack of the 11 Army, in order to achieve its goal, must be carried out as quickly as the operational situation requires. Only the speed of attack can prevent the enemy from gaining a foothold in the rear positions and systematically bringing up stronger reserves ... There are only two means for such actions: deep separation of the attacking infantry and rapid artillery pursuit by fire. Therefore, the attacking corps and divisions of the army should not think about a clearly defined task for the day, but strive by all means to advance as far as possible ... You should not expect that the attack along the entire front will go evenly ... but a faster advance of one sector of the front will facilitate the heavier and certainly slower progress in other areas and again drag them along with it." This deep separation was served by the allocation of an entire army corps to the reserve of the 11th Army.
The concentration of large enemy forces in front of the front of the 3rd Army did not go unnoticed by the Russian side. From prisoners and defectors, it became known about the offensive being prepared for approximately April 28, "the Austrians, who will be supported by the Germans." Already on April 26, the headquarters of the 3rd Army took measures to strengthen the defense at the Gorlice-Tarnów line, ordering the creation of reserves from units of the 70th and 81st Infantry Divisions. The front headquarters also took measures to return to their divisions the regiments previously allocated to other sectors, and on April 29 issued a directive to the armies on measures to form and concentrate reserves: the 3rd Caucasian Army Corps (21st and 52nd infantry divisions; a total of 23,859 combat personnel with 65 machine guns and 84 guns) entered the 3rd Army. 
The construction of defensive lines, which began during the Carpathian battle, in January 1915, was also accelerated. The operation of the fortifications was hampered in the spring by groundwater floods and landslides. On April 29, the front headquarters ordered the 3rd and 4th armies to start building the second line of defense.
The German raid into Courland diverted the attention of the Russian Headquarters from the Beskids and the Carpathians. The Austro-Hungarian Army High Command also took steps to disorientate the Russian side. On the night of April 30, the Austro-Hungarian 31st Landwehr Infantry Regiment crossed the Vistula River and, at the cost of heavy losses, captured a bridgehead in the rear of the Russian 9th Army Corps.
The concentration of forces of the Central Powers in the Gorlice sector, was known to the Russian command, which took, as far as possible, measures to strengthen the defense. For several days, the second echelon behind the positions of the Russian 10th Army Corps gathered troops equal to it in number (41,912 infantry and 985 cavalry, excluding rear services personnel), and even superior in machine guns and guns (121 and 118, respectively). This objectively reduced the superiority of the Central Powers, but the scale of this superiority was unknown to the Russian side, which played a fatal role. The headquarters of the 3rd Army and the Southwestern Front were going to repel an increased attack, similar to the February and March 1915 attempts by the Austro-Hungarian troops, possibly in the form of the last battles in the Beskids. Neither the commanders nor the fighters were ready for the fact that an entire German army would be brought into battle on a narrow sector of the front.
Battle.
May 1–2.
At dusk on May 1, the artillery of the three armies of the Central Powers opened fire on the Russian positions. By the morning of May 2, the shelling of the Gorlice sector took on the character of a hurricane. The head of the Russian 61st Infantry Division, Major General P. Simansky, noted: “Despite the well-known difficulty of loading, heavy guns, due to their large number and undoubtedly frequent installation, rattled like simple guns rattle with burst fire”. At 10:00 o'clock the German infantry went on the attack. Under the cover of dense artillery fire that destroyed trenches and shelters, some German gun crews rolled them out to support the infantry, approaching Russian positions up to 40–50 m, and fired direct fire at machine gun nests.
Several attacks were repulsed by the Russian 9th, 31st and 61st Infantry Divisions; at the same time, the 70th Infantry Division began to retreat from the right flank, which was attacked by the Austro-Hungarian 9th Corps, and against the 10th th army corps, the attacks were interspersed with new shelling, so that the trenches were completely destroyed. At 17:42, the corps commander N. Protopopov allowed the retreat to the rear position at nightfall. A brigade of the 63rd Infantry Division was introduced into the battle. However, the commander of the 3rd Army, Radko Dmitriev, “did not see sufficient motives” for retreat, ordered to continue to hold the already almost lost position and even go on the offensive from it after the approach of the divisions of the 3rd Caucasian Corps in a day or two.
However, by 20 o'clock the location of the divisions of the Russian 10th Army Corps was completely broken through, the dominant heights were captured by the Germans, and the reserves were used up. On the front of the Russian 9th Army Corps, the 70th Infantry Division was broken through and thrown back, and the 42nd Division restored the situation with difficulty. The breakthrough was carried out on the 60-km front, 2–4 km deep. 20,000 men were taken prisoners, the Germans captured 50 machine guns and 12 guns. At 21:30 Radko Dmitriev ordered the 9th Army Corps to restore the situation, the 10th Army Corps to strengthen in a new position.
May 3.
On May 3, von Mackensen united the 11th Bavarian, 20th and 119th Infantry Divisions into the corps of General of Infantry Otto von Emmich and reinforced the Guards Corps with a part of the field artillery of the 19th Infantry Division and ordered the offensive to continue. Despite the introduction of new units into battle from the Russian side, the Germans continued to successfully advance. Another 4,000 prisoners, 2 guns and 19 machine guns were captured.
On the night of May 3, Radko Dmitriev reported the situation to Ivanov, commander of the Southwestern Front; he informed Headquarters of the breakthrough of the 11th German Army and asked to send a division from the armies of the North-Western Front to the threatened area. Further withdrawal of the 10th Army Corps and the 70th Infantry Division to some extent of the Dunaets River and the beginning of the offensive of the 11th Russian Army (the 18th Army Corps was transferred from the 9th Army), during which 17 officers and 1,570 soldiers and 4 machine guns were captured. But help on the right flank was excluded: with strong attacks, the Central Powers completely fettered the units of the 4th Russian army, only the 298th Mstislavsky infantry regiment was sent from it to the southern bank of the Vistula River.
At 14:49, the Supreme Commander-in-Chief, Grand Duke Nikolai Nikolayevich, informed Ivanov about the direction of one infantry division to him and recommended that the 33rd Army Corps be transferred from the 9th Army to the 3rd Army, and in case of danger he promised to reinforce the latter with a corps of the 7th Army (located in Odessa and Sevastopol). Ivanov convinced the Headquarters of the need to keep the 33rd Corps in the 9th Army, which was preparing to go on the offensive (it still took about two weeks to transport it), and on the same day, the Chief of Staff of the Supreme Commander, Infantry General N. N. Yanushkevich agreed to go over to the offensive of the 9th Army, at the same time explaining that “for the North-Western Front, distracted at the present time by the need to stop the unfolding German offensive on Riga ... it is extremely difficult, if not impossible, to further strengthen the South-Western front."
The commander of the 3rd Army formed a group of artillery general V. Irmanov (3rd Caucasian and 10th Army Corps), who was ordered to attack the Germans.
May 4.
On May 4, the Quartermaster General of the Headquarters, Infantry General Y. Danilov, spoke by telephone with the Chief of Staff of the Southwestern Front, V. Dragomirov, to report on the situation. The latter reported that the presence of the 11th German army in the sector of the Russian 3rd Army had already been established, to which Danilov noted that there were enough reserves on the Southwestern Front that were not brought into battle (the Headquarters was completely unfamiliar with the current situation), and supported the plan for the speedy transition to the offensive of the 9th Army: "If the 9th Army really goes on the offensive without delay and achieves its goal, then it seems that even a partial withdrawal of the 3rd Army will be redeemed by this success". Dragomirov assured that the offensive would begin on May 7–8, but complained about the lack of shells.
However, the counterattack of the group of Irmanov was repelled by noon on May 4, the Germans broke through at the junction of the 3rd Caucasian and 24th army corps, other corps of the 11th army also successfully advanced.
The 3rd Army suffered losses unprecedented in severity. The head of the 61st Infantry Division, Major General P. Simansky, reported to the headquarters of the 10th Army Corps: "I testify to the unconditional valor of the troops, who fought for three days under the fire of numerous enemy heavy artillery, inflicting enormous losses and severe wounds. The 61st division faithfully fulfilled the