Once the Kingdom of Itlay entered the campaign against Austria-Hungary things looked initially to favour Italy. The Austria-Hungary's army was spread to cover its fronts with Serbia and Russia, and Italy had a numerical superiority against the Austro-Hungarian Army. However, this advantage was never fully utilized because Italian military commander Luigi Cadorna insisted on a dangerous frontal assault against Austria-Hungary in an attempt to occupy the Slovenian plateau and Ljubljana. This assault would put the Italian army not far away from Austria-Hungary's imperial capital, Vienna. After eleven failed offensives with enormous loss of life, the Italian campaign to take Vienna collapsed.
Upon entering the war, geography was also a difficulty for Italy, as its border with Austria-Hungary was along mountainous terrain. In May 1915, Italian forces at 400,000 men along the border outnumbered the Austrian and Germans almost precisely four to one. However the Austrian defenses were strong even though they were undermanned and managed to hold off the Italian offensive. The battles with the Austro-Hungarian Army along the Alpine foothills in the trench warfare there were drawn-out, long engagements with little progress.
Italian officers were poorly trained in contrast to the Austro-Hungarian and German armies, Italian artillery was inferior to the Austrian machine guns and the Italian forces had dangerously low supply of ammunition, this shortage would continually hamper attempts to make advances into Austrian territory. This combined with the constant replacement of officers by Cadorna resulted in few officers gaining the experience necessary to lead military missions.
In the first year of the war, poor conditions on the battlefield led to outbreaks of cholera causing a significant number of Italian soldiers to die. Despite these serious problems, Cadorna refused to back down the offensive. Naval battles occurred between the Italian Royal Navy and the Austro-Hungarian Navy. Italy's warships were outclassed by the Austro-Hungarian fleet and the situation was made more dire for Italy in that both the French Navy and the British Royal Navy were not sent into the Adriatic Sea. Their respective governments viewed the Adriatic as far too dangerous to operate due the concentration of the Austro-Hungarian fleet there.
Morale fell among Italian soldiers who lived a tedious life when not on the front lines: they were forbidden to enter theatres or bars even when on leave. However when battles were about to occur, alcohol was made freely available to the soldiers in order to reduce tension before the battle. In order to escape the tedium after battles, some groups of soldiers worked to create improvised brothels. In order to maintain morale, the Italian army had propaganda lectures of the importance of the war to Italy, especially in order to retrieve Trento and Trieste from Austria-Hungary. Some of these lectures were carried out by popular nationalist war proponents such as Gabriele D'Annunzio. D'Annunzio himself would participate in a number of paramilitary raids on Austrian positions along the Adriatic coastline during the war and temporaly lost his sight after a plane raid. Prominent pro-war advocate Benito Mussolini was prevented from giving lecture by the government, most likely because of his revolutionary socialist past.
The Italian government became increasingly aggravated in 1915 with the passive nature of the Serbian army which had not engaged in a serious offensive against Austria-Hungary for months.Seton-Watson, pp. 452–3 The Italian government blamed Serbian military inactiveness for allowing the Austrians to muster their armies against Italy. Cadorna suspected that Serbia was attempting to negotiate an end to fighting with Austria and addressed this to foreign minister Sidney Sonnino, who himself bitterly claimed that the Serbia was an unreliable ally. Relations between Italy and Serbia became so cold that the other Entente members were forced to abandon the idea of forming a united Balkan front against Austria-Hungary. In negotiations, Sonnino remained willing to allow Bosnia to join Serbia, but refused to discuss the fate of Dalmatia which was claimed by Italy and Pan-Slavists in Serbia.
When Serbia fell to the Austro-Hungarian and German forces in 1915, Cadorna proposed sending 60,000 men to land in Thessaloniki to help the Serbs now in exile in Greece and the Principality of Albania to fight off the opposing forces, but the Italian government's bitterness to Serbia resulted in the proposal being rejected. After 1916, the situation for Italy grew steadily worse, the Austro-Hungarian army managed to push the Italian Army back into Italy as far as Verona and Padua in their Strafexpedition. At the same time Italy faced a shortage of warships, increased attacks by submarines, soaring freight charges threatening the ability to supply food to soldiers, lack of raw materials and equipment, and Italians faced high taxes to pay for the war.
Austro-Hungarian and German forces had gone deep into northern Italian territory, and finally in November 1916, Cadorna ended offensive operations and began a defensive approach. In 1917, France, the United Kingdom and the United States offered to send troops to Italy to help it fend off the offensive of the Central Powers, but the Italian government refused, as Sonnino did not want Italy to be seen as a client state of the Allies and preferred isolation as the more brave alternative.
Italy also wanted to keep the Kingdom of Greece out of the war, as the Italian government feared that should Greece join the war on the side of the Allies, it would intend to annex Albania, which Italy wanted as its own. Fortunately for Italy, the Venizelist pro-war advocates in Greece failed to succeed in pressuring Constantine I of Greece to bring the country into the conflict, and Italian aims on Albania remained unthreatened.
The Russian Empire collapsed in a 1917 Russian Revolution, eventually resulting in the rise of the communist Bolshevik regime of Vladimir Lenin. The resulting marginalization of the Eastern Front allowed for more Austro-Hungarian and German forces arriving on the front against Italy. Internal dissent against the war grew with increasingly poor economic and social conditions in Italy due to the strain of the war. Much of the profit of the war was being made in the cities, while rural areas were losing income. The number of men available for agricultural work had fallen from 4.8 million to 2.2 million, though through the help of women, agricultural production managed to be maintained at 90 percent of its pre-war total during the war.
Many pacifist and internationalist Italian socialists turned to Bolshevism and advocated negotiations with the workers of Germany and Austria-Hungary to help end the war and bring about Bolshevik revolutions. The newspaper Avanti! of the Italian Socialist Party declared "Let the bourgeoisie fight its own war". Leftist women in northern Italian cities led protests demanding action against the high cost of living and demanding an end to the war. In Milan in May 1917, Communist revolutionaries organized and engaged in rioting, calling for an end to the war, and managed to close down factories and stop public transportation. The Italian army was forced to enter Milan with tanks and machine guns to face communists and anarchists who fought violently until 23 May when the army gained control of the city with almost fifty people killed (three of which were Italian soldiers) and over 800 people arrested.
After the Battle of Caporetto in 1917, Italian forces were forced far back into Italian territory, and the humiliation led to the arrival of Vittorio Emanuele Orlando as Prime Minister who managed to solve some of Italy's wartime problems. Orlando abandoned the previous isolationist approach to the war and increased coordination with the Allies and the use of the convoy system to fend off submarine attack, allowed Italy to be able to end food shortages from February 1918 onward, and Italy received more raw materials from the Allies. Also in 1918, began the official repression of enemy aliens and Italian socialists were increasingly repressed by the Italian government.
The Italian government was infuriated with the Fourteen Points of Woodrow Wilson, the President of the United States, as the advocation of national self-determination meant that Italy would not gain Dalmatia as had been promised in the Treaty of London. In the Parliament of Italy, nationalists condemned Wilson's fourteen points as betraying the Treaty of London, while socialists claimed that Wilson's points were valid and claimed the Treaty of London was an offense to the rights of Slavs, Greeks, and Albanians. Negotiations between Italy and the Allies, particularly the new Yugoslav delegation (replacing the Serbian delegation), agreed to a trade off between Italy and the new Kingdom of Yugoslavia, which was that Dalmatia as claimed by Italy would be accepted as Yugoslav, while Istria as claimed by the Yugoslavs would be accepted as Italian.
At the Battle of the Piave River the Italian army managed to hold off the Austro-Hungarian and German armies. The opposing armies repeatedly failed afterwards in major battles such as Battle of Asiago and the Battle of Vittorio Veneto. The Italian Army crushed the Austrian offensive in the latter battle. Austria-Hungary ended the fighting against Italy with the armistice on 4 November 1918 which ended World War I on this front (one week before the widely understood November 11th armistice on the Western front), in which the Italian Army decisively defeated the Austro-Hungarian invader.
During the war, the Italian Royal Army increased in size from 15,000 men in 1914 to 160,000 men in 1918, with 5 million recruits in total entering service during the war. This came at a terrible cost: by the end of the war, Italy had lost 700,000 soldiers and had a budget deficit of twelve billion lira. Italian society was divided between the majority pacifists who opposed Italian involvement in the war and the minority of pro-war nationalists who had condemned the Italian government for not having immediately gone to war with Austria in 1914.