Battles of the Isonzo (1915-1917): Italy's Mountain War

The Battles of the Isonzo were a series of twelve battles fought between Italy and Austria-Hungary from June 1915 to November 1917 along the Isonzo (Soča) river, in the mountains and limestone plateaus north of Trieste. They formed the core of Italy’s First World War and cost hundreds of thousands of lives for gains of only a few miles, ending in the Italian collapse at Caporetto.

Background: why Italy went to war

Italy had been formally allied to Germany and Austria-Hungary in the Triple Alliance, but stayed neutral when war broke out in 1914. In the secret Treaty of London of 26 April 1915, Britain, France and Russia promised Italy Austrian-held territory, including Trentino, South Tyrol, Trieste and parts of Dalmatia. Italy declared war on Austria-Hungary on 23 May 1915.

The new front ran for about 600 km (370 miles) along the Alps. Its eastern end, along the Isonzo, offered the only realistic route towards Trieste and on into the Habsburg heartland. The Italian chief of staff, General Luigi Cadorna, made this sector the focus of his offensives.

The ground and the defenders

The terrain favoured the defence. North of Gorizia the river runs through a deep valley overlooked by peaks such as Monte Nero (Krn) and Monte Sabotino. To the south lies the Karst (Carso) plateau, a waterless landscape of bare limestone where trenches had to be blasted from rock and shell bursts sent stone splinters flying.

The Austro-Hungarian Fifth Army, commanded from 1915 by General Svetozar Boroević, held the heights. Boroević, a Croatian-born officer, insisted on holding every metre of ground and counter-attacking quickly. His troops were usually outnumbered, but well dug in.

The eleven Italian offensives

The First Battle of the Isonzo began on 23 June 1915 and the Second followed in July. Both gained almost nothing. The Third and Fourth Battles, in October to December 1915, were equally costly. By the end of the first year Italy had already suffered heavy losses for little ground.

In May 1916 Austria-Hungary launched its own attack from the Trentino further west, the so-called Strafexpedition (punitive expedition), which was halted. On 29 June 1916 Austro-Hungarian troops used poison gas on Monte San Michele on the Karst, killing thousands of Italian soldiers.

The Sixth Battle, in August 1916, was Italy’s greatest success: troops of the Third Army under the Duke of Aosta captured Monte Sabotino and entered Gorizia on 8–9 August. The Seventh, Eighth and Ninth Battles in the autumn of 1916 pushed only a little further across the Karst.

In 1917 Cadorna attacked again. The Tenth Battle (May–June) and the Eleventh (August–September), in which General Luigi Capello’s Second Army seized much of the Bainsizza (Banjška) plateau, came close to breaking the exhausted Austro-Hungarian line.

Caporetto: the twelfth battle

Fearing a collapse of its ally, Germany sent seven divisions to reinforce the front. On 24 October 1917 a combined Austro-German force attacked near Kobarid, using gas, short bombardments and infiltration tactics. The Italian line broke, and the army retreated more than 100 km to the Piave river. This Twelfth Battle of the Isonzo is described in detail in the article on the Battle of Caporetto.

Casualties and conditions

Figures for the Isonzo battles vary considerably between sources. Italian casualties over the eleven offensives are usually estimated at well over half a million killed, wounded, missing and captured; some historians give figures closer to one million. Austro-Hungarian losses are commonly put at several hundred thousand.

Soldiers on both sides endured avalanches and frostbite in the mountains, thirst on the Karst, and disease. Italian discipline under Cadorna was notoriously harsh, including summary executions and occasional decimation of units accused of cowardice.

Key people

• Luigi Cadorna (1850–1928), Italian chief of staff, who directed the first eleven battles and was dismissed after Caporetto.
• Svetozar Boroević (1856–1920), Austro-Hungarian commander on the Isonzo, nicknamed the “Lion of the Isonzo”.
• Emanuele Filiberto, Duke of Aosta (1869–1931), commander of the Italian Third Army on the Karst, who took Gorizia.
• Luigi Capello (1859–1941), commander of the Second Army in 1917.

Timeline

• 1915, 26 April: Treaty of London commits Italy to join the Allies.
• 1915, 23 May: Italy declares war on Austria-Hungary.
• 1915, 23 June: The First Battle of the Isonzo begins.
• 1916, 29 June: Austro-Hungarian gas attack on Monte San Michele.
• 1916, 8–9 August: Italian troops enter Gorizia during the Sixth Battle.
• 1917, 12 May: The Tenth Battle opens.
• 1917, 18–19 August: The Eleventh Battle begins; Italians advance on the Bainsizza plateau.
• 1917, 24 October: The Twelfth Battle (Caporetto) breaks the Italian front.

Legacy

After the war the Isonzo valley was given to Italy, but since 1947 most of it has belonged to Yugoslavia and then Slovenia. The Kobarid Museum, the Walk of Peace trail and the vast Italian memorial at Redipuglia, which holds the remains of around 100,000 soldiers, now commemorate the battles. Italy recovered and won the Battle of Vittorio Veneto in 1918, but the memory of the Isonzo shaped bitter post-war debates in Italy.

Why it matters

The Isonzo campaign was one of the deadliest sustained struggles of the First World War, comparable in its logic of attrition to the Battle of Verdun. It drained both Italy and Austria-Hungary and helped prepare the ground for the political turmoil that followed in both countries.