Two of the three great fortresses forming the Polish triangle had now gone; Brest alone remained, and its doom was already sealed. While the Russians were preparing for a further retirement, their right, which rested on the Baltic Sea, was threatened with a new danger. On Sunday, 10th August, a German fleet tried to force a way into the Gulf of Riga; it was beaten off, but the attempt was renewed on the 15th and on the 16th. During a thick fog the Germans got into the Gulf on the 16th, and two days later tried to land troops at an unfortified port on the road to Petrograd. Four very large flat-bottomed barges, filled with troops, attempted to get ashore on the 20th; but the Russian light craft swooped down upon them, and captured or destroyed the whole of the landing force. Meanwhile a naval battle was going on throughout the length of the Gulf. The Russians lost an old gunboat, while the Germans had eight destroyers and two cruisers either sunk or put out of action, and a submarine driven ashore. On the 21st the Germans left the Gulf. Their attempt to outflank the Russian right had failed. During the previous twenty days the Germans in the centre had pushed forward no less than one hundred miles; but the Russian armies had eluded them, and no crushing battle had taken place. The Germans had not yet given up all hope of overwhelming the Russians, but for the time being they were anxious to secure a strong line on which they might maintain themselves during the coming winter. They had also a new campaign in view. Already they were thinking of forcing a way through the Balkans to Constantinople, so as to open a road to the east and fling such forces into Gallipoli as would drive the British and French into the sea. Now we must return to the doomed fortress of Brest. On the day that the German warships left the Gulf of Riga, Prince Leopold was close to the western walls of the fortress, while von Mackensen, east of the Bug, was threatening to take the forts from the rear. On 25th August Brest Litovski fell. It had held out long enough to enable the Russians to get away with the guns and supplies, and only a little wheat was left behind. Soon after the Germans entered the place a mine exploded and destroyed a thousand of their troops. [Illustration: Map illustrating the various Stages of the Russian Retreat. A, after the fall of Lemberg; B, after the fall of Warsaw; C, after the fall of Grodno; D, after the fall of Vilna.]