
Italy declared war on Austria-Hungary on May 23, 1915, entering World War I on the side of the Allies. This decision was largely influenced by the Treaty of London, signed in April 1915, which promised Italy substantial territorial gains, including control over Italian-populated lands along its border with Austria-Hungary. Italy's entry into the war opened up a new front, stretching 600 kilometers along its border with Austria-Hungary, and marked a significant development in the conflict.
| Characteristics | Values |
|---|---|
| Year Italy declared war on Austria-Hungary | 1915 |
| Date Italy declared war on Austria-Hungary | 23 May 1915 |
| Italy's allies in the war | Britain, France and Russia |
| Italy's gains from the war | Control over territory on its border with Austria-Hungary stretching from Trentino through the South Tyrol to Trieste, parts of Dalmatia, numerous islands along Austria-Hungary's Adriatic coast, the Albanian port city of Vlore and a central protectorate in Albania, and territory from the Ottoman Empire |
| Italy's motives for entering the war | To gain control of Italian-speaking territories and unite them with their cultural homeland, to gain glory and to satisfy popular feeling |
| Italy's stance at the outbreak of World War I | Neutral |
| Italy's allies before the war | Germany and Austria-Hungary (as part of the Triple Alliance) |
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What You'll Learn
- Italy declared war on Austria-Hungary on 23 May 1915
- Italy joined the Triple Entente, receiving promises of territorial gains
- Italy's entry into the war opened a new front in World War I
- Italy's irredentist movement sought to liberate Italian-speaking populations from the Austro-Hungarian Empire
- Italy's decision to enter the war was influenced by promises in the Treaty of London

Italy declared war on Austria-Hungary on 23 May 1915
The decision to join the Allies was based largely on the assurances Italy received in the Treaty of London, signed in April 1915. By its terms, Italy would receive the fulfilment of its national dream: control over territory on its border with Austria-Hungary stretching from Trentino through the South Tyrol to Trieste. The Allies also promised the Italians parts of Dalmatia and numerous islands along Austria-Hungary's Adriatic coast, as well as the Albanian port city of Vlore (Italian: Valona) and a central protectorate in Albania. Italy's entry into the war opened up a new front in World War I, stretching 600 kilometres—most of them mountainous—along Italy's border with Austria-Hungary.
Upon declaring war, the Italian army immediately advanced into the South Tyrol region and to the Isonzo River, where Austro-Hungarian troops met them with a stiff defence. The Battles of the Isonzo ensued, with the first five achieving little at the cost of 280,000 men. In late October 1917, German intervention to help Austria-Hungary resulted in a spectacular victory over the Italians in the Battle of Caporetto, during which Italian forces suffered some 300,000 casualties and were forced to retreat. By the time fighting ended on the Italian front on 4 November 1918, 615,000 Italians had been killed in action or died of wounds sustained in World War I.
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Italy joined the Triple Entente, receiving promises of territorial gains
Italy had been a member of the Triple Alliance with Germany and Austria-Hungary since 1882. However, when World War I broke out in 1914, Italy declared its neutrality, reasoning that the Triple Alliance was a defensive agreement and that its treaty partners had assumed an offensive posture with Austria-Hungary's declaration of war against Serbia.
Italy's decision to remain neutral was also influenced by its desire to gain territorial concessions from Austria-Hungary, specifically in the Trentino region and certain border areas in Friuli. These demands were supported by Germany, which hoped that concessions would prevent Italy from entering the war. However, Austria-Hungary vehemently rejected these demands, wishing to preserve its territorial integrity.
In the background, Italy had started aligning itself closer to the Entente powers, France and Great Britain, for military and economic support. On April 26, 1915, Italy negotiated the secret Pact of London, by which Great Britain, France, and Russia promised to support Italy's annexation of the frontier lands in return for entering the war on the Entente side. The Treaty of London, signed in April 1915, assured Italy of its national dream: control over territory on its border with Austria-Hungary stretching from Trentino through the South Tyrol to Trieste. It also promised Italy parts of Dalmatia, numerous islands along Austria-Hungary's Adriatic coast, and territory from the Ottoman Empire.
Thus, on May 23, 1915, Italy declared war on Austria-Hungary, joining the Triple Entente and opening up a new front in World War I. Italy's entry into the war was of decisive importance and forced Austria to concentrate its forces on its southern border.
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Italy's entry into the war opened a new front in World War I
On May 23, 1915, Italy declared war on Austria-Hungary, entering World War I on the side of the Allies—Britain, France, and Russia. Italy's entry into the war opened up a new front in World War I, stretching 600 kilometres along its border with Austria-Hungary. This border was mostly mountainous, with some areas reaching 1,000 meters in height. The topographical characteristics of the front area had a significant impact on the conduct of the war. The rocky ground, for example, made it difficult for both sides to dig trenches, and the karst rock in the Isonzo Valley posed an additional danger to the soldiers as it acted as shrapnel when grenades exploded.
Italy's decision to join the war was largely influenced by the Treaty of London, signed in April 1915, which promised Italy substantial territorial gains at the expense of Austria-Hungary. These included control over Italian-speaking territories along their border, such as Trentino, South Tyrol, and Trieste, as well as non-Italian-populated areas like the Austrian Littoral on the northern Adriatic and parts of Dalmatia. Italy's entry into the war was also driven by strong sentiments within the general population and political factions to go to war against Austria-Hungary, Italy's historical enemy.
Upon declaring war, the Italian army advanced into the South Tyrol region and the Isonzo River, where they faced stiff resistance from Austro-Hungarian troops. The Battles of the Isonzo ensued, with both sides engaging in persistent offensives and counteroffensives. The Italians suffered heavy casualties, with over 300,000 casualties in the Battle of Caporetto alone. Despite these setbacks, Italy's allies, including Britain, France, and later the United States, offered increased assistance, helping to turn the tide in the Italian campaign.
Italy's entry into World War I had significant consequences for the course of the war. It shifted Austria's focus to its southern border, opening up a new theatre of war that would be of decisive importance. Italy's participation also contributed to the collapse of the centuries-old Habsburg Empire and secured for Italy a permanent seat in the League of Nations' executive council, alongside its allies.
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Italy's irredentist movement sought to liberate Italian-speaking populations from the Austro-Hungarian Empire
Italy's unification in 1870 left many ethnic Italian speakers outside the borders of the Kingdom of Italy. This situation created Italian irredentism, a political movement that sought to bring under Italian rule various lands that were considered to be culturally, linguistically, or historically Italian. Italian irredentism was particularly focused on liberating Italian-speaking populations from the Austro-Hungarian Empire.
The Austro-Hungarian Empire included several Italian-speaking areas, such as the Italian-speaking areas of Tyrol (including Trento), Istria, Gorizia, Trieste, and the Dalmatian coastline. The Italian population in Dalmatia was concentrated in the major coastal cities, such as Split, Zadar, and Šibenik. In 1891, the Lega Nazionale Italiana ("Italian National League") was founded in Trento and Trieste to advocate for the unification of these Italian-speaking areas with Italy.
The Italian irredentist movement gained momentum after the Third Italian War of Independence in 1866, when the Veneto and Friuli regions were ceded by the Austrians to the newly formed Kingdom of Italy, but Istria and Dalmatia remained part of the Austro-Hungarian Empire. This triggered the gradual rise of Italian irredentism among many Italians in Istria, Kvarner, and Dalmatia, who demanded the unification of these areas with Italy. The Italians in these regions supported the Italian Risorgimento, which fought for the unification of Italy, and as a consequence, the Austrians saw them as enemies.
Italian irredentism played a role in Italy's decision to enter World War I on the side of the Allies in 1915. Italy had been a member of the Triple Alliance with Germany and Austria-Hungary since 1882, but when World War I broke out in 1914, Italy declared its neutrality. Italy eventually joined the Allies in 1915, with the understanding that it would receive substantial territorial gains at the expense of Austria-Hungary. The Allies promised Italy control of territory on its border with Austria-Hungary, including the Italian-speaking areas of Trentino and South Tyrol, as well as non-Italian-speaking areas such as the Austrian Littoral and parts of Dalmatia. Italy declared war on Austria-Hungary on May 23, 1915, opening up a new front in World War I.
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Italy's decision to enter the war was influenced by promises in the Treaty of London
Italy declared war on Austria-Hungary on May 23, 1915, entering World War I on the side of the Allies. When World War I broke out in 1914, Italy had declared itself neutral in the conflict, despite being a member of the Triple Alliance with Germany and Austria-Hungary.
Italy's decision to enter the war was influenced by promises made in the Treaty of London, also known as the Pact of London. Signed on April 26, 1915, it was a secret agreement between Italy, the United Kingdom, France, and Russia. The treaty aimed to bring Italy into World War I on the side of the Triple Entente, comprising France, Britain, and Russia. Italy's participation was significant due to its border with Austria-Hungary, which opened up a new theatre of war in the south.
The Treaty of London involved promises of Italian territorial expansion at the expense of Austria-Hungary, the Ottoman Empire, and colonies in Africa. Specifically, Italy was promised control of territories on its border with Austria-Hungary, stretching from Trentino through the South Tyrol to Trieste. This included both Italian-speaking regions, such as Trentino, and German-speaking areas, such as southern Tyrol. Additionally, Italy was promised parts of Dalmatia, numerous islands along the Adriatic coast, the Albanian port city of Vlore, and territories from the Ottoman Empire.
The Allies hoped that by enticing Italy to join the war with these promises, they could force the Central Powers, particularly Germany and Austria-Hungary, to divert their forces and weaken their position. Furthermore, they aimed to encourage other countries, such as Romania and Bulgaria, to join their cause. However, Italy's entry into the war did not immediately lead to a declaration of war against Germany, causing resentment among France and the UK.
The Treaty of London had far-reaching consequences, both during and after World War I. Italy's involvement in the war resulted in significant casualties, with 615,000 Italians killed or dying from wounds by the time the fighting ended on the Italian front in November 1918. Additionally, Italy suffered social unrest and economic damage due to its participation.
After the war, at the Paris Peace Conference, the United States pressured to void the treaty as it contradicted the principle of self-determination. As a result, Italy received less territorial gain than initially promised, causing resentment among Italians and fueling the rise of Benito Mussolini and his fascist movement.
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Frequently asked questions
Italy declared war on Austria-Hungary on May 23, 1915.
Italy had been a member of the Triple Alliance with Germany and Austria-Hungary since 1882 but declared its neutrality when World War I broke out in 1914. Italy eventually joined the war on the side of the Allies (Britain, France, and Russia) as they promised Italy substantial territorial gains at the cost of Austria-Hungary.
Italy's entry into the war opened up a new theatre of war in the south of the Monarchy that was to be of decisive importance for the Austro-Hungarian army. Italy's army immediately advanced into the South Tyrol region and to the Isonzo River, where Austro-Hungarian troops put up a stiff defense. Italy suffered a spectacular defeat in the Battle of Caporetto in October 1917, after which British, French, and American troops arrived in the region to offer increased assistance. By the time the fighting ended on November 4, 1918, 615,000 Italians had been killed in action or died of wounds sustained in World War I.























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