Believing that the Russians would be unable to attack them for several weeks, the Germans had left but three army corps to defend East Prussia. Imagine their surprise when, as early as 3rd August, bands of Cossacks came spurring across the border, raiding the frontier posts, and driving off their garrisons. The inhabitants of the villages were terrified at the very name of Cossack, and fled at their approach. These Cossack raids heralded the approach of two Russian armies. On 7th August a swarm of aeroplanes flew across the border near Suwalki,[35] and soon afterwards General Rennenkampf's army set foot on German soil. Rennenkampf was a dashing soldier, who had made a reputation in the war against Japan, and his army was just as eager and enthusiastic as he was. In the ranks were large numbers of young volunteers belonging to the best families of Russia, and it is said that it included some women who had cut off their hair and had enlisted as men. Rennenkampf marched north-east to strike at the railway which you see on the map running from Kowno[36] to Königsberg. His first object was to reach Insterburg, the junction of all the railways in East Prussia. If this town could be captured, Königsberg itself might be besieged. At the same time another Russian army, under General Samsonov, who had won renown as the commander of the Siberian Cossacks in the war with Japan, began to push into East Prussia from Mlava,[37] which you will find close to the border line, at the south of the Masurian Lake Region. He had a large force, probably consisting of five army corps, and his object was to march northward along the fringes of the lake district towards Königsberg. Now let us see how these two armies carried out their mission. Rennenkampf first met the enemy in force at Gumbinnen, about fifteen miles to the east of Insterburg. All round the town there are great pine woods, between which are fields of rye, studded with windmills. The Germans had entrenched themselves near the town, and had cut down thousands of trees, which they had piled up in front of their trenches to form obstacles. The battle began on Sunday morning, 16th August. Again and again the Russians charged the trenches, and again and again they were beaten back. A fierce artillery duel raged, and it was soon clear that the Russian guns and gunners could more than hold their own against the Germans. All day the white-tunicked infantry of the Tsar hammered at the German trenches in front, while their comrades were working steadily round the left flank. Towards sunset the Germans found themselves almost enveloped; they were forced to retreat, and began streaming back towards the town, with the Cossacks hard on their rear. The retreat soon became a rout, and many prisoners and machine guns were captured.