defended by a circle of forts. It fell on 17 August, along with 1,300 guns and almost 1 million shells. On 5 August his forces were consolidated into Army Group Hindenburg, which took the city of Grodno after bitter street fighting, but could not trap the retreating defenders because the rail lines lacked the capacity to bring up the needed men. They occupied Vilnius on 18 September, then halted on ground favorable for a defensive line. On 6 August, German troops under Hindenburg used chlorine gas against Russian troops defending Osowiec Fortress. The Russians demolished much of Osowiec and withdrew on 18 August. In October, Hindenburg moved headquarters to Kovno. They were responsible for 108,800 km (42,000 mi) of conquered Russian territory, which was home to three million people and became known as Ober Ost. The troops built fortifications on the eastern border while Ludendorff "with his ruthless energy" headed the civil government, using forced labor to repair the war damages and to dispatch useful products, like hogs, to Germany. A Hindenburg son-in-law, who was a reserve officer and a legal expert, joined the staff to write a new legal code. Baltic Germans who owned vast estates feted Hindenburg and he hunted their game preserves. Hindenburg would later judge German operations in 1915 to be "unsatisfactory". In his memoirs, he recounted that "[t]he Russian bear had escaped our clutches" and abandoning the Polish salient had shortened their lines substantially. Conversely, victorious Falkenhayn believed that "The Russian Army has been so weakened by the blows it has suffered that Russia need not be seriously considered a danger in the foreseeable future". The Russians replaced their experienced supreme commander, Grand Duke Nicholas Nikolaevich, a man whose skill Hindenburg held in high regard, with the incompetent Tsar. 1916 Brusilov Offensive In the spring of 1916, the Central Powers experienced a military catastrophe in the East that left Germany bearing much of the war effort until the end of hostilities. On 4 June, the Russian Army began a massive offensive along of the southwestern front in present-day western Ukraine. In the ensuing onslaught, four armies commanded by General Aleksei Brusilov overwhelmed entrenchments that the Austro-Hungarians long regarded as impregnable. Probing assault troops located three weak spots which then were struck in force. In nine days they captured more than 200,000 men and 200 guns, and pushed into open country. Under Hindenburg's command, Ober Ost