Italy, although a member of the Triple Alliance with Austria-Hungary and Germany, had remained neutral in 1914. It had not been consulted beforehand and the treaty did not require it to participate in an attack on Serbia. Austria-Hungary was Italy’s traditional enemy and incorporating the 800,000 Italian speakers around Trieste and in the Trentino region of the Hapsburg empire into Italy was the main objective of Italian nationalists.
Italy had joined the Triple Alliance in 1882 for defensive reasons: it was too weak to fight Austria-Hungary on its own and it considered the German army to be the best in Europe. However, Italy and Austria-Hungary both strengthened their navies and the fortifications on their mutual frontier whilst supposedly allied.
Once war between the Central Powers of Germany and Austria-Hungary and the Entente of France, Russia and the United Kingdom had begun, neutrality was the obvious and most popular course for Italy. It had a long coastline and imported most of its wheat and coal by sea, making it very vulnerable to attack by the Royal Navy.
Italy, however, was biding its time. the question was whether it could best achieve its objectives by joining the Central Powers, the Entente or staying neutral. The Treaty of London of 26 April 1915 required it to join the Entente within a month.
The Italian speaking regions of Trieste and Trentino would become Italian. It would also obtain defensible frontiers and dominance of the Adriatic by the incorporation of German, Croatian and Slovenian speaking areas and receive colonies in Africa and Asia Minor.
The Italians expected wrongly that their objectives could be gained by a quick war against Austria-Hungary, so declared war initially on only it.
On 10 May, a week after Italy had renounced the Triple Alliance, a Naval Convention was signed between France, Italy and the UK. The French navy and the RN would support the Italian navy until the end of the war, unless the Austro-Hungarian fleet was destroyed earlier. The Italians would command in the Adriatic and the French elsewhere in the Mediterranean, except in Ottoman waters, where the British would command until the end of the Dardanelles operation.
The Italian navy would be reinforced by four British battleships, four British light cruisers, 12 French destroyers, as many torpedo boats, submarines and minesweepers as the French could spare, including at least six submarines and six French seaplanes, with a seaplane carrier if possible.
This resulted in the following balance of power in the Adriatic:
|Type||Austria-Hungary||Italy||Lent by UK & France|
|Fast light cruisers||5||3||2|
|Old light cruisers||2||3||2|
Source: Naval Staff Monograph (Historical) 1923 vol. viii The Mediterranean 1914-1915, p. 143.
These figures exclude an Austro-Hungarian and two Italian dreadnoughts that were close to completion and some German submarines that were assembled at the Austro-Hungarian base at Pola for operations in the Dardanelles.
The British ships would come from the Dardanelles, where they would be replaced by four French cruisers and an increase in the French battleship force to six.
The Italian handed their declaration of war to the Austro-Hungarians on 23 May: it came into effect at midnight. The British squadron was at the Italian naval base of Taranto by the morning of 27 May.
 The above is based on D. Stevenson, 1914-1918: The History of the First World War (London: Allen Lane, 2004), pp. 110-13.
 Naval Staff Monograph (Historical) 1923 vol. viii The Mediterranean 1914-1915. pp. 141-42.
 Ibid. Footnotes 1, 2 & 11, p. 143.
 J. S. Corbett, H. Newbolt, Naval Operations, 5 vols. (London: HMSO, 1938). vol. ii, p. 396.
 Naval Staff vol. viii. pp. 144-45.