Italy Joined the Triple Entent

Italy had been allied with the German and Austro-Hungarian Empires since 1882 as part of the Triple Alliance. However, the nation had its own designs on Austrian territory in Trentino, Istria and Dalmatia. Rome had a secret 1902 pact with France, effectively nullifying its alliance. At the start of hostilities, Italy refused to commit troops, arguing that the Triple Alliance was defensive in nature, and that Austria–Hungary was an aggressor. The Austro-Hungarian government began negotiations to secure Italian neutrality, offering the French colony of Tunisia in return. The Allies made a counter-offer in which Italy would receive the Southern Tyrol, Venezia Giulia and territory on the Dalmatian coast after the defeat of Austria-Hungary. This was formalized by the Treaty of London. Further encouraged by the Allied invasion of Turkey in April 1915, Italy joined the Triple Entente and declared war on Austria-Hungary on May 23. Fifteen months later Italy declared war on Germany.

Militarily, the Italians had numerical superiority. This advantage, however, was lost, not only because of the difficult terrain in which fighting took place, but also because of the strategies and tactics employed. Field Marshal Luigi Cadorna, a staunch proponent of the frontal assault, had dreams of breaking into the Slovenian plateau, taking Ljubljana and threatening Vienna. It was a Napoleonic plan, which had no realistic chance of success in an age of barbed wire, machine guns, and indirect artillery fire, combined with hilly and mountainous terrain.

On the Trentino front, the Austro-Hungarians took advantage of the mountainous terrain, which favoured the defender. After an initial strategic retreat, the front remained largely unchanged, while Austrian Kaiserschützen and Standschützen engaged Italian Alpini in bitter hand-to-hand combat throughout the summer. The Austro-Hungarians counter attacked in the Altopiano of Asiago, towards Verona and Padua, in the spring of 1916, (Strafexpedition), but made little progress.

Beginning in 1915, the Italians under Cadorna mounted eleven offensives on the Isonzo front along the Isonzo River, north east of Trieste. All eleven offensives were repelled by the Austro-Hungarians, who held the higher ground. In the summer of 1916, the Italians captured the town of Gorizia. After this minor victory, the front remained static for over a year, despite several Italian offensives. In the autumn of 1917, thanks to the improving situation on the Eastern front, the Austro-Hungarian troops received large numbers of reinforcements, including German Stormtroopers and the elite Alpenkorps. The Central Powers launched a crushing offensive on 26 October 1917, spearheaded by the Germans. They achieved a victory at Caporetto. The Italian army was routed and retreated more than 100 kilometres (60 mi.) to reorganize, stabilising the front at the Piave River. Since in the Battle of Caporetto Italian Army had heavy losses, the Italian Government called to arms the so called '99 Boys (Ragazzi del '99), that is, all males who were 18 years old. In 1918, the Austro-Hungarians failed to break through, in a series of battles on the Piave River and, finally being decisively defeated in the Battle of Vittorio Veneto in October of that year. Austria-Hungary surrendered in early November 1918.

The Ottoman Empire Joined the Central Powers

The Ottoman Empire joined the Central Powers in the war, the secret Ottoman-German Alliance having been signed in August 1914. It threatened Russia's Caucasian territories and Britain's communications with India via the Suez Canal. The British and French opened overseas fronts with the Gallipoli (1915) and Mesopotamian campaigns. In Gallipoli, Turkey successfully repelled the British, French and Australian and New Zealand Army Corps (ANZACs). In Mesopotamia, by contrast, after the disastrous Siege of Kut (1915–16), British Imperial forces reorganized and captured Baghdad in March 1917. Further to the west, in the Sinai and Palestine Campaign, initial British setbacks were overcome when they captured Jerusalem in December 1917. The Egyptian Expeditionary Force, under Field Marshal Edmund Allenby, broke the Ottoman forces at the Battle of Megiddo in September 1918.

Russian armies generally had the best of it in the Caucasus. Enver Pasha, supreme commander of the Turkish armed forces, was ambitious and dreamed of conquering central Asia. 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 with a string of victories. In 1917, Russian Grand Duke Nicholas assumed command of the Caucasus front. Nicholas planned a railway from Russian Georgia to the conquered territories, so that fresh supplies could be brought up for a new offensive in 1917. However, in March 1917, (February in the pre-revolutionary Russian calendar), the Czar was overthrown in the February Revolution and the Russian Caucasus Army began to fall apart.

The army corps of Armenian volunteer units realigned under the command of General Tovmas Nazarbekian, with Dro as a civilian commissioner of the Administration for Western Armenia. The front line had three main divisions commanded by Movses Silikyan, Andranik, and Mikhail Areshian. Another regular unit was under Colonel Korganian. More than 40,000 men in Armenian partisan guerrilla detachments accompanied the main units.

Instigated by the Arab bureau of the British Foreign and Commonwealth Office, the Arab Revolt was a major cause of the Ottoman Empire's defeat. The revolts started with the help of Britain in June 1916 at the Battle of Mecca, led by Sherif Hussein of Mecca, and ended with the Ottoman surrender of Damascus. Fakhri Pasha, the Ottoman commander of Medina, resisted for more than two and half years during the Siege of Medina.

Along the border of Italian Libya and British Egypt, the Senussi tribe, 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 deal with the Senussi. Their rebellion was finally crushed in mid-1916.

War in the Balkans

Faced with Russia, 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, however, succeeded in driving them from the country by the end of 1914. For the first ten months of 1915, Austria-Hungary used most of its military reserves to fight Italy. German and Austro-Hungarian diplomats, however, scored a coup by persuading Bulgaria to join in attacking Serbia. The Austro-Hungarian provinces of Slovenia, Croatia and Bosnia provided troops for Austria-Hungary, invading Serbia as well as fighting Russia and Italy. Montenegro allied itself with Serbia.

Serbia was conquered in a little more than a month. The attack began in October, when the Central Powers launched an offensive from the north; four days later the Bulgarians joined the attack from the east. The Serbian army, fighting on two fronts and facing certain defeat, retreated into Albania, halting only once to make a stand against the Bulgarians. The Serbs suffered defeat near modern day Gnjilane in the Battle of Kosovo. Montenegro covered the Serbian retreat toward the Adriatic coast in the Battle of Mojkovac in 6–7 January 1916, but ultimately the Austrians conquered Montenegro, too. Serbian forces were evacuated by ship to Greece.

In late 1915, a Franco-British force landed at Salonica in Greece, to offer assistance and to pressure the government to declare war against the Central Powers. Unfortunately for the Allies, the pro-German King Constantine I dismissed the pro-Allied government of Eleftherios Venizelos, before the Allied expeditionary force could arrive.

After conquest, Serbia was divided between Austro-Hungary and Bulgaria. Bulgarians commenced bulgarization of the Serbian population in their occupation zone, banishing Serbian Cyrillic and the Serbian Orthodox Church. After forced conscription of the Serbian population into the Bulgarian army in 1917, the Toplica Uprising began. Serbian rebels liberated for a short time the area between the Kopaonik mountains and the South Morava river. The uprising was crushed by joint efforts of Bulgarian and Austrian forces at the end of March 1917.

The Macedonian Front proved static for the most part. Serbian forces retook part of Macedonia by recapturing Bitola on 19 November 1916. Only at the end of the conflict were the Entente powers able to break through, after most of the German and Austro-Hungarian troops had withdrawn. The Bulgarians suffered their only defeat of the war at the Battle of Dobro Pole but days later, they decisively defeated British and Greek forces at the Battle of Doiran, avoiding occupation. Bulgaria signed an armistice on 29 September 1918.

Naval War of World War I

Naval War of World War I: At the start of the war, the German Empire had cruisers scattered across the globe, some of which were subsequently used to attack Allied merchant shipping. The British Royal Navy systematically hunted them down, though not without some embarrassment from its inability to protect Allied shipping. For example, the German detached light cruiser SMS Emden, part of the East-Asia squadron stationed at Tsingtao, seized or destroyed 15 merchantmen, as well as sinking a Russian cruiser and a French destroyer. However, the bulk of the German East-Asia squadron—consisting of the armoured cruisers Scharnhorst and Gneisenau, light cruisers Nürnberg and Leipzig and two transport ships—did not have orders to raid shipping and was instead underway to Germany when it encountered elements of the British fleet. The German flotilla, along with Dresden, sank two armoured cruisers at the Battle of Coronel, but was almost destroyed at the Battle of the Falkland Islands in December 1914, with only Dresden and a few auxiliaries escaping, but at the Battle of Más a Tierra these too were destroyed or interned.

Soon after the outbreak of hostilities, Britain initiated a naval blockade of Germany. The strategy proved effective, cutting off vital military and civilian supplies, although this blockade violated generally accepted international law codified by several international agreements of the past two centuries. Britain mined international waters to prevent any ships from entering entire sections of ocean, causing danger to even neutral ships. Since there was limited response to this tactic, Germany expected a similar response to its unrestricted submarine warfare.

The 1916 Battle of Jutland (German: Skagerrakschlacht, or "Battle of the Skagerrak") developed into the largest naval battle of the war, the only full-scale clash of battleships during the war, and one of the largest in history. It took place on 31 May – 1 June 1916, in the North Sea off Jutland. The Kaiserliche Marine's High Seas Fleet, commanded by Vice Admiral Reinhard Scheer, squared off against the Royal Navy's Grand Fleet, led by Admiral Sir John Jellicoe. The engagement was a stand off, as the Germans, outmanoeuvred by the larger British fleet, managed to escape and inflicted more damage to the British fleet than they received. Strategically, however, the British asserted their control of the sea, and the bulk of the German surface fleet remained confined to port for the duration of the war.

German U-boats attempted to cut the supply lines between North America and Britain. The nature of submarine warfare meant that attacks often came without warning, giving the crews of the merchant ships little hope of survival. The United States launched a protest, and Germany modified its rules of engagement. After the notorious sinking of the passenger ship RMS Lusitania in 1915, Germany promised not to target passenger liners, while Britain armed its merchant ships, placing them beyond the protection of the "cruiser rules" which demanded warning and placing crews in "a place of safety" (a standard which lifeboats did not meet). Finally, in early 1917 Germany adopted a policy of unrestricted submarine warfare, realizing the Americans would eventually enter the war. Germany sought to strangle Allied sea lanes before the U.S. could transport a large army overseas, but were only able to maintain five long range U-boats on station, to limited effect.

The U-boat threat lessened in 1917, when merchant ships entered convoys escorted by destroyers. This tactic made it difficult for U-boats to find targets, which significantly lessened losses; after the introduction of hydrophone and depth charges, accompanying destroyers might attack a submerged submarine with some hope of success. The convoy system slowed the flow of supplies, since ships had to wait as convoys were assembled. The solution to the delays was an extensive program to build new freighters. Troop ships were too fast for the submarines and did not travel the North Atlantic in convoys. The U-boats had sunk almost 5,000 Allied ships, at a cost of 178 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.

Early Stages of World War I

Trench warfare begins

Military tactics before World War I had failed to keep pace with advances in technology. These changes resulted in the building of impressive defence systems, which out of date tactics could not break through for most of the war. Barbed wire was a significant hindrance to massed infantry advances. Artillery, vastly more lethal than in the 1870s, coupled with machine guns, made crossing open ground very difficult. The Germans introduced poison gas; it soon became used by both sides, though it never proved decisive in winning a battle. Its effects were brutal, causing slow and painful death, and poison gas became one of the most-feared and best-remembered horrors of the war. Commanders on both sides failed to develop tactics for breaching entrenched positions without heavy casualties.

In time, however, technology began to produce new offensive weapons, such as the tank. Britain and France were its primary users; the Germans employed captured Allied tanks and small numbers of their own design.

After the First Battle of the Marne, both Entente and German forces began a series of outflanking manoeuvres, in the so-called "Race to the Sea". Britain and France soon found themselves facing entrenched German forces from Lorraine to Belgium's Flemish coast. Britain and France sought to take the offensive, while Germany defended the occupied territories; consequently, German trenches were generally much better constructed than those of their enemy. Anglo-French trenches were only intended to be "temporary" before their forces broke through German defences. Both sides attempted to break the stalemate using scientific and technological advances. On 22 April 1915 at the Second Battle of Ypres, the Germans (in violation of the Hague Convention) used chlorine gas for the first time on the Western Front. Algerian troops retreated when gassed and a six kilometre (four mile) hole opened in the Allied lines that the Germans quickly exploited, taking Kitcheners' Wood. Canadian soldiers closed the breach at the Second Battle of Ypres. At the Third Battle of Ypres, Canadian and ANZAC troops took the village of Passchendaele.

On Saturday, 1 July 1916, the British Army endured the bloodiest day in its history, suffering 57,470 casualties, including 19,240 dead on the first day of the Battle of the Somme. Most of the casualties occurred in the first hour of the attack. The entire Somme offensive cost the British Army almost half a million men.

Neither side proved able to deliver a decisive blow for the next two years, though protracted German action at Verdun throughout 1916, combined with the bloodletting at the Somme, brought the exhausted French army to the brink of collapse. Futile attempts at frontal assault came at a high price for both the British and the French poilu (infantry) and led to widespread mutinies, especially during the Nivelle Offensive.

Throughout 1915–17, the British Empire and France suffered more casualties than Germany, due both to the strategic and tactical stances chosen by the sides. At the strategic level, while the Germans only mounted a single main offensive at Verdun, the Allies made several attempts to break through German lines. At the tactical level, Ludendorff's doctrine of "elastic defence" was well suited for trench warfare. This defence had a relatively lightly defended forward position and a more powerful main position farther back beyond artillery range, from which an immediate and powerful counter-offensive could be launched.

Ludendorff wrote on the fighting in 1917,

The 25th of August concluded the second phase of the Flanders battle. It had cost us heavily ... The costly August battles in Flanders and at Verdun imposed a heavy strain on the Western troops. In spite of all the concrete protection they seemed more or less powerless under the enormous weight of the enemy’s artillery. At some points they no longer displayed the firmness which I, in common with the local commanders, had hoped for. The enemy managed to adapt himself to our method of employing counter attacks ... I myself was being put to a terrible strain. The state of affairs in the West appeared to prevent the execution of our plans elsewhere. Our wastage had been so high as to cause grave misgivings, and had exceeded all expectation.

On the battle of the Menin Road Ridge, Ludendorff wrote,

Another terrific assault was made on our lines on the 20 September ... The enemy’s onslaught on the 20th was successful, which proved the superiority of the attack over the defence. Its strength did not consist in the tanks; we found them inconvenient, but put them out of action all the same. The power of the attack lay in the artillery, and in the fact that ours did not do enough damage to the hostile infantry as they were assembling, and above all, at the actual time of the assault.

Around 1.1 to 1.2 million soldiers from the British and Dominion armies were on the Western Front at any one time. A thousand battalions, occupying sectors of the line from the North Sea to the Orne River, operated on a month-long four-stage rotation system, unless an offensive was underway. The front contained over 9,600 kilometres (5,965 mi) of trenches. Each battalion held its sector for about a week before moving back to support lines and then further back to the reserve lines before a week out-of-line, often in the Poperinge or Amiens areas.

In the 1917 Battle of Arras, the only significant British military success was the capture of Vimy Ridge by the Canadian Corps under Sir Arthur Currie and Julian Byng. The assaulting troops were able for the first time to overrun, rapidly reinforce and hold the ridge defending the coal-rich Douai plain.

Opening Hostilities of World War I

Confusion among the Central Powers

The strategy of the Central Powers suffered from miscommunication. Germany had promised to support Austria-Hungary’s invasion of Serbia, but interpretations of what this meant differed. Previously tested deployment plans had been replaced early in 1914, but never tested in exercises. Austro-Hungarian leaders believed Germany would cover its northern flank against Russia. Germany, however, envisioned Austria-Hungary directing the majority of its troops against Russia, while Germany dealt with France. This confusion forced the Austro-Hungarian Army to divide its forces between the Russian and Serbian fronts.

On Wednesday, 9 September 1914, the Septemberprogramm, a plan which detailed Germany's specific war aims and the conditions that Germany sought to force upon the Allied Powers, was outlined by German Chancellor Theobald von Bethmann Hollweg.

African campaigns

Some of the first clashes of the war involved British, French and German colonial forces in Africa. On 7 August, French and British troops invaded the German protectorate of Togoland. On 10 August German forces in South-West Africa attacked South Africa; sporadic and fierce fighting continued for the remainder of the war. The German colonial forces in German East Africa, led by Colonel Paul Emil von Lettow-Vorbeck, fought a guerilla warfare campaign for the duration of World War I, surrendering only two weeks after the armistice took effect in Europe.

Serbian campaign

The Serbian army fought the Battle of Cer against the invading Austro-Hungarians, beginning on 12 August, occupying defensive positions on the south side of the Drina and Sava rivers. Over the next two weeks Austrian attacks were thrown back with heavy losses, which marked the first major Allied victory of the war and dashed Austro-Hungarian hopes of a swift victory. As a result, Austria had to keep sizeable forces on the Serbian front, weakening its efforts against Russia.

German forces in Belgium and France

At the outbreak of the First World War, the German army (consisting in the West of seven field armies) executed a modified version of the Schlieffen Plan, designed to quickly attack France through neutral Belgium before turning southwards to encircle the French army on the German border. The plan called for the right flank of the German advance to converge on Paris and initially, the Germans were very successful, particularly in the Battle of the Frontiers (14–24 August). By 12 September, the French with assistance from the British forces halted the German advance east of Paris at the First Battle of the Marne (5–12 September). The last days of this battle signified the end of mobile warfare in the west. The French offensive into Germany launched on August 7 with the Battle of Mulhouse had limited success.

In the east, only one Field Army defended East Prussia and when Russia attacked in this region it diverted German forces intended for the Western Front. Germany defeated Russia in a series of battles collectively known as the First Battle of Tannenberg (17 August – 2 September), but this diversion exacerbated problems of insufficient speed of advance from rail-heads not foreseen by the German General Staff. The Central Powers were thereby denied a quick victory and forced to fight a war on two fronts. The German army had fought its way into a good defensive position inside France and had permanently incapacitated 230,000 more French and British troops than it had lost itself. Despite this, communications problems and questionable command decisions cost Germany the chance of obtaining an early victory.

Asia and the Pacific

New Zealand occupied German Samoa (later Western Samoa) on 30 August. On 11 September, the Australian Naval and Military Expeditionary Force landed on the island of Neu Pommern (later New Britain), which formed part of German New Guinea. Japan seized Germany's Micronesian colonies and, after the Battle of Tsingtao, the German coaling port of Qingdao in the Chinese Shandong peninsula. Within a few months, the Allied forces had seized all the German territories in the Pacific; only isolated commerce raiders and a few holdouts in New Guinea remained.