Although Italy declared itself neutral in 1914 and for a while was clearly watching how thewar developed, the secret Treaty of London, signed on 26 April 1915, brought it onto the side of the Entente. On 23 May 1915, Italy declared war on Austria-Hungary. Germany immediately severed diplomatic relations, but it was not until 28 August 1916 that Italy also declared war on Germany. The Treaty promised Italy territories in the Trentino, Tirol, Trieste and all of Istria, Dalmatia, Valona and the Dodecanese Islands, as well as parts of Asia-Minor.

Once engaged, many Italian efforts to break through the Austria-Hungarian forces in the area of the Roiver Isonzo proved insufficient despite very high losses.

British and French forces were deployed as a result of a decision by the Supreme War Council during a crisis after the Italians had been pushed back 40 miles west, across the River Tagliamento to the River Piave, by a German-managed attack at Caporetto (the Twelfth Battle of the Isonzo) in late 1917.

British promises of assistance to Italy had been made as early as the Rome Conference of January 1917, at which Lloyd George proposed British reinforcement. Typically, he did this without referring to the CIGS, Sir William Robertson. Lloyd George suggested that if the Italians had to defend against enemy attack, they would be reinforced by British and French heavy artillery. Robertson went to see for himself in March 1917 and concluded that the Italian tactics and administration were fragile and that infantry would probably also be needed. As a result of these discussions, ten newly-raised 6-inch howitzer batteries (total 40 guns) were sent to the Carso sector. There were at this time only 152 such batteries on the Western Front.

On 24 October 1917 a combined Austro-German force attacked after a heavy bombardment, with gas, against Italian troops in the upper valley of the Isonzo. A precipitate retreat followed, with the enemy in pursuit as the broken Italian forces fled westwards. The Italian Government immediately pleaded for Allied help. An initial British force, consisting of the XIV Corps under the command of the Earl of Cavan, with the 23rd and 41st Divisions, was despatched and began to arrive on 11 November 1917. On the 13th, General Sir Herbert Plumer arrived and assumed command of the British forces in Italy. 5th, 7th, and 48th Divisions eventually also joined the force in Italy, as did XI Corps headquarters.

By 16 November, the enemy offensive had come to an end. Only the early British artillery had played a part up to this point. The Italian Army had lost 10,000 dead, 30,000 wounded and 265,000 prisoners in three weeks. They began to strengthen the new defensive positions, and to reorganise. German interest in the Italian sector waned, especially as they became locked into the mighty offensives of spring 1918 on the Western Front. Two British Divisions - 5th and 41st - returned to France in February 1918, as did Plumer, and XI Corps. Lord Cavan commanded the British force thereafter.

The Austrians attacked strongly in June 1918 in the Asiago, but were repulsed. Late in October, British forces attacked along the Piave and succeeded in pushing the enemy back into the Vittorio Veneto, until an Armistice was called by the defeated Austrians early in November 1918. By then, units of the British Army had also advanced in the Trentino and were the first such troops to enter the home soil of a European enemy during the war.

At peak in January 1918, there were almost 114,000 British troops in Italy, but on average the force was only 78,000. In total, 90 officers and 1,200 other ranks died in Italy (in addition to 759 who died of disease or injury). 4,700 men were wounded and 50,000 reported sick at some point, of which 11,500 had influenza.

The Italian Army faced its Austrian opponents all the way along the northern border, much of which runs through the very mountainous country of the Tirol and Southern Alps. However, most of the serious fighting between these belligerents was in the area of the River Isonzo, which lies some 50 miles East of Venice towards Trieste. Today much of the area is in Slovenia. The Eleven Battles of the Isonzo were large-scale and fierce, but ultimately had little strategic result.

The main areas of British interest are