On October 24, 1917, German and Austro-Hungarian units attacked the Italian positions at Caporetto on the upper Isonzo river, and after a short bombardment with gas and high explosive they broke through the line in a few hours. The attack was the twelfth battle on the Isonzo in two and a half years, and it was the first in which German divisions took the leading part. The front collapsed. The Italian front had been static until then.
The methods were new for this front. The attackers used small groups to find weak points and move around the strong ones, and they had better artillery support and gas shells, and the Italian reserves were placed too far back to plug the gaps, and the orders given to the local commanders were unclear. The result was a general retirement that turned into a rout, with units losing touch with each other and prisoners taken in very large numbers. The retreat went on for two weeks.
The cost was heavy. The Italians lost about 10,000 killed, some 30,000 wounded and around 300,000 taken prisoner, and they abandoned guns and stores, and they fell back more than 100 kilometres to the line of the Piave river, where they stopped. The commander, Luigi Cadorna, was replaced, and a new government took office under Vittorio Emanuele Orlando, and the army was reorganised and its treatment of the soldiers improved.
The defeat had a political effect as well. The allies sent troops to Italy, and the Italian government presented the retreat as a national effort against invasion, and the line on the Piave held through the Austrian offensive of June 1918. The line held. The Italians attacked across the river in October 1918 and won the battle of Vittorio Veneto. Caporetto is remembered in Italy as the country's worst defeat of the war.