But the most ominous German advance was far to the north, where Von Buelow was profiting by the fall of Kovno, marching on Mitau and Riga, and threatening both to cut the railway between Vilna and Petrograd and confine the Russian retreat to congested and narrow lines of communication along which they could not escape. This northern advance was accompanied by a naval offensive in the Baltic, designed to seize Riga and turn the line of the Dvina on which the Russians hoped to stand in the last resort. Fortunately this part of the campaign broke down before matters had reached their worst on land. It looked like a naval operation planned, or at least attempted, by soldiers professionally incapable of grasping the elementary principles of naval or amphibious warfare. After an unsuccessful attack on the southern inlet to the Gulf of Riga on 10 August, the Germans during a thick fog on the 17th sought to land troops at Pernau in large flat-bottomed barges without having secured command of the sea; and the entire landing-force was captured or destroyed. Simultaneously the Russian Fleet engaged the Germans, who had eight destroyers and two cruisers sunk or put out of action; the only Russian vessel lost was an old gunboat. The Dvina lines were not to be turned by strategy like this, and Russia was henceforth free from naval interference until her sailors played her false. Von Buelow was still, however, to be reckoned with, and he was the substantial danger. On 28 August he began his movement against the Dvina, which would, if successful, cut off all the Russian armies from direct communication with Petrograd. The blow was struck at Friedrichstadt, where the river is crossed by the only practicable road between Riga and Jacobstadt, but the design was to turn the whole front as far as Dvinsk; and Von Buelow held out to his troops the alluring prospect of winter quarters in Riga and a march on Petrograd in the spring. On 3 September the left bank was cleared for some miles, but all attempts to cross were frustrated. The out-march on the extreme German left had failed, and the critical point moved south towards Vilna. The danger here was serious enough, for the depletion of the Russian forces and length of their line had left a gap between Dvinsk and Vilna, and into this gap the Germans thrust a huge cavalry force which more nearly turned the Russian line than any other movement in the campaign.