[Illustration: Map to illustrate von Hindenburg's Advance to the Niemen and the Battle of Augustovo.] Rennenkampf was unable to offer much opposition to von Hindenburg as he pushed forward, nor would he have resisted him if he could. His object was to lure von Hindenburg on towards the Niemen, where he felt sure he could put an end to his advance. If he could force the Germans to retreat, he would be able to fall upon their rear as they marched back along the narrow roads with the deadly swamps and quagmires around them, and revenge Tannenberg. He therefore let the enemy come on, and only delayed him from time to time by a little rearguard fighting. The German troops which travelled by railway moved fast. On 20th September the siege of Ossowietz began, and next day the main bodies of the enemy reached the Niemen at three points, marked A, B, and C on the diagram. Rennenkampf by this time had got most of his men over the broad stream, and they were now lying in deep trenches on the low eastern shore. He had received large reinforcements, and he was now confident that he could prevent the Germans from crossing. On the morning of 26th September von Hindenburg's heavy howitzers began to throw their shells across the river at B, while his engineers built pontoon bridges. As soon as a bridge was completed, concealed Russian guns blew it to pieces. All day long the howitzers boomed, but there was no reply from the Russian side. At nightfall von Hindenburg felt sure that he had driven his enemy out of their trenches, and that next day he might safely attempt to cross the river. On the morning of the 27th bridges were again built and swung across the stream. The Russians waited until the Germans were on them, and then their guns smashed them to fragments. There was terrible loss on the German side, and nowhere could they make headway. At all points along the river they were held up in the same way. Meanwhile the siege of Ossowietz had hopelessly failed: in the spongy moss surrounding the "island" of solid ground on which the fortress is built no firm positions could be found for the big guns. The Russians were too strong for him, and on Sunday, 27th September, von Hindenburg gave the order to retreat. He now realized that he could not cross the Niemen, and that even if he could, his success would not force the Russians to withdraw troops from Galicia. The retreat was a difficult matter; but von Hindenburg, as you know, was a master of marsh warfare. Only in the centre, where he had to cross the swampy country to the east and south-east of Suwalki, was he in difficulties.