Von Mackensen began the action by an advance on Gorlice, and Dmitrieff was forced to weaken his centre in order to strengthen his left wing. Meanwhile the real attack was beginning farther north. Midway between Gorlice and Tarnow you will see a break on the Russian front. It was against this point that von Mackensen made his great effort. On the morning of 1st May hundreds of his big guns began to roar, and soon shells were falling fast and thick on the Russian trenches. It is said that no less than 700,000 shells were hurled against the Russian positions on that day. Nothing could live in the whirlwind of death, and speedily the Russian trenches were wiped out, and thousands of Russians with them. Meanwhile pontoons were pushed across the river, and as soon as the bombardment ceased wave after wave of Germans in close order surged forward. By the next day they had broken through the Russian line, and the whole of the front was turned. There was nothing for it but swift retreat. Nor were the Russians able to make a stand until they were twenty miles from the position which they had occupied all winter. On the little river Wisloka they halted, and turned their faces to the foe. The line of the river gave them but a poor defence; but it was necessary that they should hold back the enemy if Brussilov's army, which lay along the foothills of the Carpathians, was to escape. By this time the famous Caucasian corps had been hurried up to stem the German torrent. These devoted men, though they had no heavy guns, defied the terrible artillery fire of the enemy, and at length managed to get to grips with him. They fought with supreme valour; they captured one of the German batteries, took 7,000 prisoners, and slew many thousands more. Not until they had lost 10,000 men did they perceive that no human valour could avail against Mackensen's merciless guns. For five priceless days they held out, and then were forced to retreat towards the little river Wistok. Soon the retreat became something like a rout, and Brussilov's army was in deadly peril. For a moment it looked as if von Mackensen was about to roll up the two halves of Dmitrieff's army and achieve another Sedan. But the Russians managed to push out strong forces towards Sanok and check the advancing enemy. The five days' stand on the Wisloka and this new movement enabled Brussilov, after much desperate fighting, to withdraw his troops from the foothills, and to retreat eastwards. Many of his men who were south of the Dukla and Lupkow Passes were cut off and made prisoners.