In Europe their defence was materially assisted by the British and French attacks in Artois and Champagne and by the needs of Mackensen's offensive in the Balkans. To both areas troops were diverted from the German front in Russia, and the centre was especially denuded. No advantage was, however, taken of this weakness, partly because of Russia's general debility and partly because what efforts she could afford were required for the defence of the Dvina and for the sympathetic activity of Ivanov in Galicia, which was the nearest approach Russia could make to intervention in the Balkans. The German attack on the line of the Dvina was not merely intended to fend off a Russian attack in the centre; it had also the positive aim of securing Riga and comfortable winter quarters for the German army in the north. Riga, however, was not an easy nut to crack; its flank was defended by the sea, immediately south of it were marshes across which only causeways ran, and to the east stretched the formidable obstacle of the Dvina. Roads and rails for the most part crossed it at Dvinsk, and the southern approaches to Dvinsk itself lay through land and water as intricately mixed as in the Masurian mazes of East Prussia. But on Dvinsk the German attack was concentrated, and after a preliminary failure on 25 September a week's bombardment and assault began on 3 October. The siege guns which had been so fatal at Kovno and elsewhere were brought up against a minor fortress and failed. Ruszky was in command, and he took care to keep the howitzers out of range of the city by an arc of far-flung trenches which the numerous scattered lakes saved from outflanking. Illukst was at one time taken by the Germans but found of little value for the larger purpose; and German prisoners complained that Dvinsk, which they failed to take, had cost them more than all the greater fortresses they had captured. In the third week of October Hindenburg transferred his efforts back to Riga, where he met with little better success. He got as far as Olai on the direct route from Mitau, and even secured a foothold on Dahlen Island in the river south-east of Riga; but these successes profited him no more than the capture of Illukst. On 7 November the Russians recaptured Olai, and on the 10th, with the help of their fleet, drove back the Germans, who had advanced along the coast, beyond Shlock and Kemmern and Kish, extending their lines to Ragassem and Kalnzem. In the same month a similar Russian counter-offensive recaptured Illutsk and pushed the Germans farther away from Dvinsk (see Map, p. 274).