World War I
Troopships were too fast for the submarines and did not travel the North Atlantic in convoys. The U-boats sunk more than 5,000 Allied ships, at the cost of 199 submarines.
World War I also saw the first use of aircraft carriers in combat, with HMS Furious launching Sopwith Camels in a successful raid against the Zeppelin hangars at Tondern in July 1918, as well as blimps for antisubmarine patrol.

Southern theatres: 

War in the Balkans: Faced with Russia in the east, Austria-Hungary could spare only one-third of its army to attack Serbia. After suffering heavy losses, the Austrians briefly occupied the Serbian capital, Belgrade. A Serbian counter-attack in the Battle of Kolubara succeeded in driving them from the country by the end of 1914. For the first 10 months of 1915, Austria-Hungary used most of its military reserves to fight Italy. German and Austro-Hungarian diplomats scored a coup by persuading Bulgaria to join the attack on Serbia. The Austro-Hungarian provinces of Slovenia, Croatia and Bosnia provided troops for Austria-Hungary. Montenegro allied itself with Serbia.

Bulgaria declared war on Serbia on 14 October 1915 and joined in the attack by the Austro-Hungarian army under Mackensen's army of 250,000 that was already underway. Serbia was conquered in a little more than a month, as the Central Powers, now including Bulgaria, sent in 600,000 troops in total. The Serbian army, fighting on two fronts and facing certain defeat, retreated into northern Albania. The Serbs suffered defeat in the Battle of Kosovo. Montenegro covered the Serbian retreat toward the Adriatic coast in the Battle of Mojkovac on 6–7 January 1916, but ultimately the Austrians also conquered Montenegro. The surviving Serbian soldiers were evacuated to Greece. After the conquest, Serbia was divided between Austro-Hungary and Bulgaria.
In late 1915, a Franco-British force landed at Salonica in Greece to offer assistance and to pressure its government to declare war against the Central Powers. However, the pro-German King Constantine I dismissed the pro-Allied government of Eleftherios Venizelos before the Allied expeditionary force arrived.
The Macedonian front was at first mostly static. French and Serbian forces retook limited areas of Macedonia by recapturing Bitola on 19 November 1916 following the costly Monastir offensive, which brought stabilisation of the front.

Serbian and French troops finally made a breakthrough in September 1918 in the Vardar offensive, after most German and Austro-Hungarian troops had been withdrawn. The Bulgarians were defeated at the Battle of Dobro Pole, and by 25 September British and French troops had crossed the border into Bulgaria proper as the Bulgarian army collapsed. Bulgaria capitulated four days later, on 29 September 1918. The German high command responded by despatching troops to hold the line, but these forces were too weak to re-establish a front.
The disappearance of the Macedonian front meant that the road to Budapest and Vienna was now opened to Allied forces. Hindenburg and Ludendorff concluded that the strategic and operational balance had now shifted decidedly against the Central Powers and, a day after the Bulgarian collapse, insisted on an immediate peace settlement.

Ottoman Empire: The Ottomans threatened Russia's Caucasian territories and Britain's communications with India via the Suez Canal. The Ottoman Empire took advantage of the European powers' preoccupation with the war and conducted large-scale ethnic cleansing of the Armenian, Greek, and Assyrian Christian populations—the Armenian genocide, Greek genocide, and Sayfo respectively.
The British and French opened overseas fronts with the Gallipoli (1915) and Mesopotamian campaigns (1914). In Gallipoli, the Ottoman Empire successfully repelled the British, French, and Australian and New Zealand Army Corps (ANZACs). In Mesopotamia, by contrast, after the defeat of the British defenders in the siege of Kut by the Ottomans (1915–1916), British Imperial forces reorganised and captured Baghdad in March 1917. The British were aided in Mesopotamia by local Arab and Assyrian fighters, while the Ottomans employed local Kurdish and Turcoman tribes.
The Suez Canal was defended from Ottoman attacks in 1915 and 1916; in August 1916, a German and Ottoman force was defeated at the Battle of Romani by the ANZAC Mounted Division and the 52nd (Lowland) Infantry Division. Following this victory, an Egyptian Expeditionary Force advanced across the Sinai Peninsula, pushing Ottoman forces back in the Battle of Magdhaba in December and the Battle of Rafa on the border between the Egyptian Sinai and Ottoman Palestine in January 1917.

Russian armies generally had success in the Caucasus campaign. Enver Pasha, supreme commander of the Ottoman armed forces, dreamed of re-conquering central Asia and areas that had been previously lost to Russia. He was, however, a poor commander. He launched an offensive against the Russians in the Caucasus in December 1914 with 100,000 troops, insisting on a frontal attack against mountainous Russian positions in winter. He lost 86% of his force at the Battle of Sarikamish. General Yudenich, the Russian commander from 1915 to 1916, drove the Turks out of most of the southern Caucasus.

The Ottoman Empire, with German support, invaded Persia (modern Iran) in December 1914 to cut off British and Russian access to petroleum reservoirs around Baku. Persia, ostensibly neutral, had long been under British and Russian influence. The Ottomans and Germans were aided by Kurdish and Azeri forces, together with a large number of major Iranian tribes, while the Russians and British had the support of Armenian and Assyrian forces. The Persian campaign lasted until 1918 and ended in failure for the Ottomans and their allies. However, the Russian withdrawal from the war in 1917 led Armenian and Assyrian forces to be cut off from supply lines, outnumbered, outgunned and isolated, forcing them to fight and flee towards British lines in northern Mesopotamia.
The Arab Revolt, instigated by the British Foreign Office, started in June 1916 with the Battle of Mecca, led by Sharif Hussein. The Sharif declared the independence of the Kingdom of Hejaz and, with British assistance, conquered much of Ottoman-held Arabia, resulting finally in the Ottoman surrender of Damascus. Fakhri Pasha, the Ottoman commander of Medina, resisted for more than 2+1⁄2 years during the siege of Medina before surrendering in January 1919.
The Senussi tribe, along the border of Italian Libya and British Egypt, incited and armed by the Turks, waged a small-scale guerrilla war against Allied troops. The British were forced to dispatch 12,000 troops to oppose them in the Senussi campaign. Their rebellion was finally crushed in mid-1916.
Total Allied casualties on the Ottoman fronts amounted to 650,000 men. Total Ottoman casualties were 725,000, with 325,000 dead and 400,000 wounded.

Italian Front: Though Italy joined the Triple Alliance in 1882, a treaty with its traditional Austrian enemy was so controversial that subsequent governments denied its existence and the terms were only made public in 1915. This arose from nationalist designs on Austro-Hungarian territory in Trentino, the Austrian Littoral, Rijeka and Dalmatia, considered vital to secure the borders established in 1866. In 1902, Rome secretly had agreed with France to remain neutral if the latter was attacked by Germany, effectively nullifying its role in the Triple Alliance.

When the war began in 1914, Italy argued the Triple Alliance was defensive and it was not obliged to support an Austrian attack on Serbia. Opposition to joining the Central Powers increased when Turkey became a member in September, since in 1911 Italy had occupied Ottoman possessions in Libya and the Dodecanese islands. To secure Italian neutrality, the Central Powers offered them Tunisia, while in return for an immediate entry into the war, the Allies agreed to their demands for Austrian territory and sovereignty over the Dodecanese. Although they remained secret, these provisions were incorporated into the April 1915 Treaty of London; Italy joined the Triple Entente and, on 23 May, declared war on Austria-Hungary, followed by Germany fifteen months later.
The pre-1914 Italian army was short of officers, trained men, adequate transport and modern weapons; by April 1915, some of these deficiencies had been remedied but it was still unprepared for the major offensive required by the Treaty of London. The advantage of superior numbers was offset by the difficult terrain; much of the fighting took place high in the Alps and Dolomites, where trench lines had to be cut through rock and ice and keeping troops supplied was a major challenge. These issues were exacerbated by unimaginative strategies and tactics. Between 1915 and 1917, the Italian commander, Luigi Cadorna, undertook a series of frontal assaults along the Isonzo, which made little progress and cost many lives; by the end of the war, Italian combat deaths totalled around 548,000.