The war began with Russian invasions of East Prussia, and of Galicia (the Austro-Hungarian province to the south of Poland and north of the Carpathian Mountains). The Germans had defeated the invasion of East Prussia, but the Austrians had been forced back to the Carpathians after the disastrous battles of Lemberg. A German attack on Warsaw in the autumn of 1914 had briefly allowed the Austrians to restore the situation, but by the spring of 1915 they were once again fighting in the Carpathians and faced a real danger that the Russians might break through into Hungary.
The Chief of the General Staff, General Conrad von Hötzendorf, devised the plan that would retrieve the situation, but it would require German troops. He called for four German divisions to be moved to the quiet western end of the Carpathian Front, where the front line turned north. This German force would break through the Russian lines and advance east behind the Russian armies in the Carpathians, forcing them to retreat or risk surrender.
The Austrian plan was accepted by the German High Command. General Falkenhayn decided to move an entire German Army (four corps, or eight divisions), to the sector of the front line that ran north from Gorlice, at the edge of the Carpathians, to Tarnow.
This was the Eleventh Army, under General von Mackensen. He was given the Guards, X, XLI and a Composite Corps, all moved in secrecy from the Western Front. The gas attack that led to the second battle of Ypres was one of a series of diversions launched to hide this movement. Mackensen was also given command of the Austrian troops allocated to the offensive, the VI corps and a Hungarian cavalry division. This army took up position behind the line of the Austrian Fourth Army, which then moved north to let the Germans take over the line. On 28 April the Germans were in place.
The Russians were massively outnumbered between Tarnow and Gorlice. Von Mackensen had 170,000 men, with 702 field guns and nearly 300 heavy guns. In the area to be attacked, the Russians had two divisions from General Radko-Dmitriev's Third Army.
The German plan was for a simple frontal assault, supported by a heavy artillery bombardment. It was thus very different from the more ambitious plans for envelopments and double envelopments that had previously dominated German thinking. It was a type of attack that would have failed on the western front, but the Russian lines between Gorlice and Tarnow were much weaker than the French or British lines in the west.
At 6 am on 2 May a four hour bombardment began. This was the heaviest yet seen on the Eastern Front, and destroyed the Russian defences. At 10 a.m. the first wave of 30,000 German and Austrian infantry attacked, and by the end of the day had captured the Russian first and second lines.
On 4 May a Russian counterattack, by III Caucasian Corps, failed, and the Germans broke out into open country. They made rapid progress to the east, threatening the entire Russian Carpathian Front. By the end of the first week of the offensive, the Germans had captured 140,000 prisoners and 100 guns, and the Russian Third Army had been destroyed. Most of its divisions were down to 1,000 men, less than 10% of their full strength. On 10 May the Austrians advancing on the German right forced their way across the river San at Sanok, and began to advance towards the fortress of Przemysl.
The German and Austrians continued to advance throughout the summer. The three Russian armies on the Carpathians were forced to retreat towards Lemberg, which itself fell on 22 June. Przemysl had been evacuated on 1 June, after an attempt to defend the San at Jaroslaw failed. The Germans then turned north, and began an advance to the east of Warsaw, while a second German attack (Twelfth Army), from the north, forced the Russians to abandon Warsaw on 5 August. On 25 August Brest-Litovsk fell to the Germans.
By the middle of September the Russians had been forced back to a line that ran from Lithuania south to the Pripet Marshes and the Rumanian border. Russian Poland had been lost, and any direct threat to German or to the Austro-Hungarian Empire was gone. The Russian commander in chief in the west, the Grand Duke Nicholas, had conducted a skilful retreat and had preserved a large part of the Russian army, but on 21 August Tsar Nicholas II transferred the Grand Duke to the Caucasus Front and took direct command of the armies. This established a link between the Tsar and the progress of the war that would play a significant role in reducing his popularity over the next two years.
The only negative element of the campaign from the German point of view was the increasing weakness of their Austro-Hungarian allies. They had lost over one million men since the start of 1915, and were becoming increasingly dependent on German aid to maintain their war effort. The process that saw the Austro-Hungarian Empire go from being Germany's almost equal ally to being their costly dependent was well under way.
Fonte:
http://www.historyofwar.org/articles/battles_gorlice_tarnow.html
