Shortly thereafter, Hindenburg and Ludendorff played a key role in the Central Powers' Gorlice–Tarnów Offensive. After the Austro-Hungarian fortress of Przemyśl fell on 23 March, Austria-Hungary's high command pushed for a joint strike on the Russian right flank that could potentially drive their forces out of the Carpathians. Agreeing to the proposal, Falkenhayn moved OHL east to the castle of Pless while forming Army Group von Mackensen from a new German Eleventh Army and the Austro-Hungarian Fourth Army. As Field Marshal August von Mackensen broke through Russian lines between Gorlice and Tarnów, Hindenburg's Ninth and Tenth Army launched diversionary attacks that threatened Riga in the north. In one of the war's most successful cavalry actions, three cavalry divisions swept east into Courland, the barren, sandy region near the Baltic coast. The cavalry's gains were held by Hindenburg's new Nieman army, named after the river. In June, the Supreme Army Command ordered Hindenburg to launch a frontal attack in Poland toward the Narew River north of Warsaw. Hindenburg created Army Group Gallwitz, named after its commander. Von Gallwitz was one of many able commanders selected by Hindenburg, who stayed at the new army's headquarters to be available if needed. (When Berlin approved the new army group, it became Twelfth Army.) The army group broke through the Russian lines after a brief, but intense, bombardment directed by Lieutenant Colonel Georg Bruchmüller, an artillery genius recalled from medical retirement. One-third of the opposing Russian First Army were casualties in the first five hours. From then on Hindenburg often called on Bruchmüller. The Russians withdrew across the Narev River. However, steamroller frontal attacks cost dearly: by 20 August Gallwitz had lost 60,000 men. Evacuation of Poland As the Russians withdrew from the Polish Salient, Falkenhayn insisted on pursuing them into Lithuania. However, Hindenburg and Ludendorff were dissatisfied with this plan. Hindenburg would later claim that he saw it as "a pursuit in which the pursuer gets more exhausted than the pursued". On 1 June, Hindenburg's Nieman and Tenth Armies spearheaded attacks into Courland in an attempt to pocket the defenders. Ultimately, this plan was foiled by the prudent commander of the Fifth Russian Army who defied orders by withdrawing into defensible positions shielding Riga. Despite the setback in Latvia, Hindenburg and Ludendorff continued to rack up victories on the Eastern Front. The German Tenth Army besieged Kovno, a Lithuanian city on the Nieman River