When war broke out in August 1914 between the Central Powers (Triple Alliance) of Germany, Austria-Hungary and later Turkey and the Entente of Russia, France, Great Britain and its Empire, Italy, a member of the Triple Alliance, stayed silent.
Ostensibly, allied to the Central Powers, Italy played a waiting game more interested in annexing land from the Austro-Hungarian Empire on its northern frontier.
Through diplomatic channels, the Allies promised Italy substantial territorial gains and this led to the secret Treaty of London with Great Britain, France, and Russia, signed on April 26, 1915. Under this treaty, Italy’s obligations to the Triple Alliance were negated. The promised territorial gains were the Italian-populated and speaking Trentino and Trieste (both under Austro-Hungarian control) and the South Tirol, Gorizia, Istria, and northern Dalmatia.
Italy’s entry to the war brought in another 900,000 men and opened a third front in the war against the Central Powers. However, the Italian army was poorly led and poorly equipped. The Commander, Cordona, was inept and lacking in combat experience.
After three years of hard and attritional fighting, often in mountainous terrain, the front ebbed and flowed with several battles of the Isonzo, offensives at Asiago and Piave all proving bloody and inconclusive.
However, on October 24th 1917 at Caporetto (12th battle of Isonzo) the Austro-Hungarian Army led and supported by Germany broke the Italian front and advanced up to 12 miles in one day. The Italian army melted away after suffering heavy casualties, over 200,000 were taken prisoner and some 400,000 men deserted.
In November 1917, Cordona was removed from command and replaced by Diaz who organised a new defensive line on the Piave. Stalemate fighting continued throughout 1918 and the Germans removed their divisions from the Italian front to bolster their Spring Offensive in the west.
In October 1918, bolstered by the arrival of British and French divisions, the Italians counter attacked and the Austro-Hungarian line collapsed under the onslaught. Trent and Trieste were captured on November 3rd and on November 4th, Austro-Hungary signed an Armistice bringing the fighting to an end, just one week before fighting ceased on the Western Front.
