In the meantime, in April 1915 Italy had concluded with Britain, France, and Russia the secret Treaty of London, which promised the Italians Trent and Trieste plus other lands at Austro-Hungarian and Turkish expense. In May the Italians formally declared war on Austria-Hungary, and a new front was added along the Austro-Italian frontier at the head of the Adriatic.
Since much of this front was mountainous, action was largely confined to some sixty miles along the Isonzo River, where for two years there was a series of bloody but indecisive engagements that pinned down several hundred thousand Austrian troops. Then in the late autumn of 1917, with Russia already beaten, came a blow that very nearly knocked Italy out.
The Germans and Austrians broke through at Caporetto and sent the Italians into retreat across the Venetian plains. French and British reinforcements were hastily rushed across the Alps, but what did most to stop the Austro-Germans was the grave difficulty of supplying their armies in such a rapid advance. The Italians were finally able to hold along the line of the Piave River.