Great Britain, France, and Russia concluded on April 26, 1915, the secret Treaty of London with Italy, inducing the latter to discard the obligations of the Triple Alliance and to enter the war on the side of the Allies by the promise of territorial aggrandizement at Austria-Hungary’s expense. Italy was offered not only the Italian-populated Trentino and Trieste but also South Tirol (to consolidate the Alpine frontier), Gorizia, Istria, and northern Dalmatia. On May 23, 1915, Italy accordingly declared war on Austria-Hungary.

The Italian commander, General Luigi Cadorna, decided to concentrate his effort on an offensive eastward from the province of Venetia across the comparatively low ground between the head of the Adriatic and the foothills of the Julian Alps; that is to say, across the lower valley of the Isonzo (Soc̆a) River. Against the risk of an Austrian descent on his rear from the Trentino (which bordered Venetia to the northwest) or on his left flank from the Carnic Alps (to the north), he thought that limited advances would be precaution enough.

The Italians’ initial advance eastward, begun late in May 1915, was soon halted, largely because of the flooding of the Isonzo, and trench warfare set in. Cadorna, however, was determined to make progress and so embarked on a series of persistent renewals of the offensive, known as the Battles of the Isonzo. The first four of these (June 23–July 7; July 18–August 3; October 18–November 4; and November 10–December 2) achieved nothing worth the cost of 280,000 men; and the fifth (March 1916) was equally fruitless. The Austrians had shown on this front a fierce resolution that was often lacking when they faced the Russians. In mid-May 1916 Cadorna’s program was interrupted by an Austrian offensive from the Trentino into the Asiago region of western Venetia. Though the danger of an Austrian breakthrough from the mountainous borderland into the Venetian plain in the rear of the Italians’ Isonzo front was averted, the Italian counteroffensive in mid-June recovered only one-third of the territory overrun by the Austrians north and southwest of Asiago. The Sixth Battle of the Isonzo (August 6–17), however, did win Gorizia for the Italians. On August 28 Italy declared war on Germany. The next three months saw three more Italian offensives on the Isonzo, none of them really profitable. In the course of 1916 the Italians had sustained 500,000 casualties, twice as many as the Austrians, and were still on the Isonzo.

Serbia and the Salonika expedition, 1915–17

Austria’s three attempted invasions of Serbia in 1914 had been brusquely repulsed by Serbian counterattacks. By the summer of 1915 the Central Powers were doubly concerned to close the account with Serbia, both for reasons of prestige and for the sake of establishing secure rail communications with Turkey across the Balkans. In August, Germany sent reinforcements to Austria’s southern front; and, on September 6, 1915, the Central Powers concluded a treaty with Bulgaria, whom they drew to their side by the offer of territory to be taken from Serbia. The Austro-German forces attacked southward from the Danube on October 6; and the Bulgars, undeterred by a Russian ultimatum, struck at eastern Serbia on October 11 and at Serbian Macedonia on October 14.

The western Allies, surprised in September by the prospect of a Bulgarian attack on Serbia, hastily decided to send help through neutral Greece’s Macedonian port of Salonika, relying on the collusion of Greece’s pro-Entente prime minister, Eleuthérios Venizélos. Troops from Gallipoli, under the French general Maurice Sarrail, reached Salonika on October 5, but on that day Venizélos fell from power.

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The Allies advanced northward up the Vardar into Serbian Macedonia but found themselves prevented from junction with the Serbs by the westward thrust of the Bulgars. Driven back over the Greek frontier, the Allies were merely occupying the Salonika region by mid-December. The Serbian Army, meanwhile, to avoid double envelopment, had begun an arduous winter retreat westward over the Albanian mountains to refuge on the island of Corfu. English relief worker Mabel St. Clair Stobart, who had been commissioned a major in the Serbian army, led the First Serbian-English Field Hospital (Front) during the retreat. Stobart achieved some renown for the feat, as her unit was one of the few to reach the Albanian coast without suffering any losses or desertions.

In the spring of 1916 the Allies at Salonika were reinforced by the revived Serbs from Corfu as well as by French, British, and some Russian troops, and the bridgehead was expanded westward to Vodena (Edessa) and eastward to Kilkis; but the Bulgars, who in May obtained Fort Rupel (Klidhi, on the Struma) from the Greeks, in mid-August not only overran Greek Macedonia east of the Struma but also, from Monastir (Bitola), invaded the Florina region of Greek Macedonia, to the west of the Allies’ Vodena wing. The Allied counteroffensive took Monastir from the Bulgars in November 1916, but more ambitious operations, from March to May 1917, proved abortive. The Salonika front was tying down some 500,000 Allied troops without troubling the Central Powers in any significant way.