Tension existed in Europe especially in the troubled Balkan regions in the southeast of Europe. A number of alliances involved European powers, Ottoman Empire, Russia and other parties had existed for years, but political instability in the Balkans threatened to destroy these agreements, till the spark of the World War I was ignited in Sarajevo, Bosnia, where Archduke Franz Ferdinand and his wife Sophie was shot by Serbian nationalist Gaverilo Princip.

The Austro-Hungarian authorities encouraged the subsequent anti-Serb riots in Sarajevo, in which Bosnian Croats killed 2 Bosnian Serbs and damaged numerous Bosnian Serb assets. violent actions against Serbs were also organized outside Sarajevo, in other Austro-Hungarian controlled in Bosnia and Herzegovina.

By 20 July, the Germans had retreated across the Marne to their starting lines, having achieved little, and the German Army never regained the initiative.

On 23 July, Austria-Hungary delivered to Serbia the July Ultimatum, a series of ten demands that were made intentionally unacceptable, in an effort to provoke a war with Serbia.

Serbia decreed general mobilization on the 25th. Serbia accepted all of the terms of the ultimatum except for article six, which demanded that Austrian delegates be allowed in Serbia for the purpose of participation in the investigation into the assassination. Following this, Austria broke off diplomatic relations with Serbia and, the next day, ordered a partial mobilization.

Finally, on 28 July 1914, a month after the assassination, Austria-Hungary declared war on Serbia.

On 29 July, Russia, in support of Serbia, declared partial mobilization against Austria-Hungary.

Russia, in support of Serbia, declared partial mobilization against Austria-Hungary. German Chancellor Bethmann-Hollweg waited until the 31st for an appropriate response, when Germany declared Erklärung des Kriegszustandes or State of Danger of War.

On 1 August, Wilhelm ordered General Helmuth von Moltke the Younger to "march the whole of the army to the East" after being wrongly informed that the British would remain neutral if France was not attacked. Moltke told the Kaiser that attempting to redeploy a million men was unthinkable, and that making it possible for the French to attack the Germans "in the rear" would prove disastrous. Yet Wilhelm insisted that the German army should not march into Luxembourg until he received a telegram sent by his cousin George V, who made it clear that there had been a misunderstanding. Eventually the Kaiser told Moltke, "Now you can do what you want."

Germany entered into World War I on August 1, 1914, when it declared war on Russia.

Russian plans for the start of the war called for simultaneous invasions of Austrian Galicia and East Prussia. Although Russia's initial advance into Galicia was largely successful, it was driven back from East Prussia by Hindenburg and Ludendorff at the battles of Tannenberg and the Masurian Lakes in August and September 1914.

On 3 August Germany declared war on France; on the same day, they sent the Belgian government an ultimatum demanding unimpeded right of way through any part of Belgium, which was refused. Early on the morning of 4 August, the Germans invaded; King Albert ordered his military to resist and called for assistance under the 1839 Treaty of London.

Britain demanded Germany comply with the Treaty and respect Belgian neutrality; it declared war on Germany at 19:00 UTC on 4 August 1914.

On 10 August, German forces in South-West Africa attacked South Africa; sporadic and fierce fighting continued for the rest of the war. The German colonial forces in German East Africa, led by Colonel Paul von Lettow-Vorbeck, fought a guerrilla warfare campaign during World War I and only surrendered two weeks after the armistice took effect in Europe.

The Battle of Kolubara was fought between Austria-Hungary and Serbia in November and December 1914, during the Serbian Campaign of World War I.

The Battle of Cer was a military campaign fought between Austria-Hungary and Serbia in August 1914, starting three weeks into the Serbian Campaign, the initial military action of the First World War. It took place around Cer Mountain and several surrounding villages, as well as the town of Šabac. The battle was a part of the first Austro-Hungarian invasion of Serbia, began on the night of 15 August when elements of the Serbian 1st Combined Division encountered Austro-Hungarian outposts that had been established on the slopes of Cer Mountain earlier in the invasion. The clashes that followed escalated into a battle for control over several towns and villages near the mountain, especially Šabac.

The initial German advance in the West was very successful: by the end of August the Allied left, which included the British Expeditionary Force (BEF), was in full retreat; French casualties in the first month exceeded 260,000, including 27,000 killed on 22 August during the Battle of the Frontiers.

Because mighty Russia support Serbia, and that will be a real threat, Austria-Hungary to declare war until its leaders reserved assurance from a great ally, German leader Kaisar Wilhelm II that Germany will support them. Austrian leaders had fear that Russian intervention will involve others, France and possibly Great Britain as well.

New Zealand occupied German Samoa (later Western Samoa) on 30 August 1914.

Von Kluck (German Leader) used this freedom to disobey orders, opening a gap between the German armies as they closed on Paris. The French and British exploited this gap to halt the German advance east of Paris at the First Battle of the Marne from 5 to 12 September and push the German forces back some 50 km (31 mi).

After the First Battle of the Marne (5–12 September 1914), Allied and German forces unsuccessfully tried to outflank each other, a series of manoeuvres later known as the "Race to the Sea". By the end of 1914, the opposing forces were left confronting each other along an uninterrupted line of entrenched positions from Alsace to Belgium's North Sea coast.

The Czechoslovak Legion fought on the side of the Entente. Its goal was to win support for the independence of Czechoslovakia. The Legion in Russia was established in September 1914.

On 28 October, the German cruiser SMS Emden sank the Russian cruiser Zhemchug in the Battle of Penang. Japan seized Germany's Micronesian colonies and, after the Siege of Tsingtao, the German coaling port of Qingdao on the Chinese Shandong peninsula.

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 ten months of 1915, Austria-Hungary used most of its military reserves to fight Italy.

In Gallipoli, the Ottoman Empire successfully repelled the British, French, and Australian and New Zealand Army Corps (ANZACs).

Both sides tried to break the stalemate using scientific and technological advances. On 22 April 1915, at the Second Battle of Ypres, the Germans (violating the Hague Convention) used chlorine gas for the first time on the Western Front. Several types of gas soon became widely used by both sides, and though it never proved a decisive, battle-winning weapon, poison gas became one of the most-feared and best-remembered horrors of the war.

Beginning in 1915, the Italians under Cadorna mounted eleven offensives on the Isonzo front along the Isonzo (Soča) River, northeast of Trieste. Of this eleven offensives, five were won by Italy, three remained inconclusive, and other three were repelled by the Austro-Hungarians, who held the higher ground.

In May, the Central Powers achieved a remarkable breakthrough on Poland's southern frontiers with their Gorlice–Tarnów Offensive. On 5 August, they captured Warsaw and forced the Russians to withdraw from Poland.

Bulgaria declared war on Serbia on 12 October 1915 and joined in the attack by the Austro-Hungarian army under Mackensen's army of 250,000 that was already underway.

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 pro-German King Constantine I dismissed the pro-Allied government of Eleftherios Venizelos before the Allied expeditionary force arrived. The friction between the King of Greece and the Allies continued to accumulate with the National Schism, which effectively divided Greece between regions still loyal to the king and the new provisional government of Venizelos in Salonica. After intense negotiations and an armed confrontation in Athens between Allied and royalist forces (an incident known as Noemvriana), the King of Greece resigned and his second son Alexander took his place

After conquest, Serbia was divided between Austro-Hungary and Bulgaria.

By the spring of 1915, the Russians had retreated to Galicia.

Montenegro covered the Serbian retreat towards the Adriatic coast in the Battle of Mojkovac in 6–7 January 1916, but ultimately the Austrians also conquered Montenegro. The surviving Serbian soldiers were evacuated by ship to Greece.

In February 1916 the Germans attacked French defensive positions at the Battle of Verdun, lasting until December 1916. The Germans made initial gains, before French counter-attacks returned matters to near their starting point. Casualties were greater for the French, but the Germans bled heavily as well, with anywhere from 700,000 to 975,000 casualties suffered between the two combatants. Verdun became a symbol of French determination and self-sacrifice.

By mid-1916, two years of war had decimated the Russian economy. It triggered downturns in agrarian production, triggered problems in the transportation network, fuelled currency inflation and created critical food and fuel shortages in the cities.

The Battle of Jutland (German: Skagerrakschlacht, or "Battle of the Skagerrak") in May/June 1916 developed into the largest naval battle of the war. It was the only full-scale clash of battleships during the war, and one of the largest in history.

The Arab Revolt, instigated by the Arab bureau of the British Foreign Office, started June 1916 with 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 before surrendering in January 1919.

The Battle of the Somme was an Anglo-French offensive of July to November 1916. The opening day of the offensive (1 July 1916) was the bloodiest day in the history of the British Army, suffering 57,470 casualties, including 19,240 dead. The entire Somme offensive cost the British Army some 420,000 casualties. The French suffered another estimated 200,000 casualties and the Germans an estimated 500,000.

Romania had been allied with the Central Powers since 1882. When the war began, however, it declared its neutrality, arguing that because Austria-Hungary had itself declared war on Serbia, Romania was under no obligation to join the war. On 4 August 1916, Romania and the Entente signed the Political Treaty and Military Convention, that established the coordinates of Romania's participation in the war. In return, it received the Allies' formal sanction for Transylvania, Banat and other territories of Austria-Hungary to be annexed to Romania. The action had large popular support.

In the summer of 1916, after the Battle of Doberdò, the Italians captured the town of Gorizia. After this victory, the front remained static for over a year, despite several Italian offensives, centred on the Banjšice and Karst Plateau east of Gorizia.

On 27 August 1916, the Romanian Army launched an attack against Austria-Hungary, with limited Russian support. The Romanian offensive was initially successful in Transylvania.

Tanks were developed by Britain and France and were first used in combat by the British during the Battle of Flers–Courcelette (part of the Battle of the Somme) on 15 September 1916, with only partial success. However, their effectiveness would grow as the war progressed; the Allies built tanks in large numbers, whilst the Germans employed only a few of their own design, supplemented by captured Allied tanks.

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.

The Battle of Buchares was the last battle of the Romanian Campaign of 1916 in World War I, in which the Central Powers' combatants, led by General Erich von Falkenhayn, occupied the Romanian capital and forced the Romanian Government, as well as the remnants of the Romanian Army to retreat to Moldavia and re-establish its capital at Iaşi.

On 12 December 1916, after ten brutal months of the Battle of Verdun and a successful offensive against Romania, Germany attempted to negotiate a peace with the Allies. However, this attempt was rejected out of hand as a "duplicitous war ruse".

Further to the west, the Suez Canal was defended from Ottoman attacks in 1915 and 1916; in August, 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.

The British naval blockade began to have a serious impact on Germany. In response, in February 1917, the German General Staff convinced Chancellor Theobald von Bethmann-Hollweg to declare unrestricted submarine warfare, with the goal of starving Britain out of the war.

In Mesopotamia, by contrast, after the defeat of the British defenders in the Siege of Kut by the Ottomans (1915–16), British Imperial forces reorganised and captured Baghdad in March 1917.

In March and April 1917, at the First and Second Battles of Gaza, German and Ottoman forces stopped the advance of the Egyptian Expeditionary Force, which had begun in August 1916 at the Battle of Romani.

Wilson (USA presedint) called for war on Germany on 2 April 1917, which the US Congress declared 4 days later.

Protracted action at Verdun throughout 1916, combined with the bloodletting at the Somme, brought the exhausted French army to the brink of collapse. Futile attempts using frontal assault came at a high price for both the British and the French and led to the widespread French Army Mutinies, after the failure of the costly Nivelle Offensive of April–May 1917.

On 3 May 1917, during the Nivelle Offensive, the French 2nd Colonial Division, veterans of the Battle of Verdun, refused orders, arriving drunk and without their weapons. Their officers lacked the means to punish an entire division, and harsh measures were not immediately implemented. The French Army Mutinies eventually spread to a further 54 French divisions, and 20,000 men deserted.

In two days the British and Indian infantry, supported by a creeping barrage, broke the Ottoman front line and captured the headquarters of the Eighth Army (Ottoman Empire) at Tulkarm, the continuous trench lines at Tabsor, Arara, and the Seventh Army (Ottoman Empire) headquarters at Nablus.

Robert Nivelle was removed from command by 15 May, replaced by General Philippe Pétain, who suspended bloody large-scale attacks.

Greece officially joined the war on the side of the Allies in June 1917.

The last large-scale offensive of this period was a British attack (with French support) at Passchendaele (July–November 1917). This offensive opened with great promise for the Allies, before bogging down in the October mud. Casualties, though disputed, were roughly equal, at some 200,000–400,000 per side.

Czechoslovak Legion saw action for the first time on July 2, 1917. That’s when a detachment of 3,500 from the unit stormed the Austrian trenches at Zborov in present-day Ukraine. Czechoslovak Legion troops defeated the Austro-Hungarian army at the Ukrainian village of Zborov.

World War I was a major disaster for the Russian Empire, leading to its collapse in October 1917. The 1.7 million wartime casualties were just the start of even more carnage. Even though Russia exited the war with the Treaty of Brest-Litovsk on March 3, 1918, the Civil War plunged the country into even greater violence and destruction.

The Central Powers launched a crushing offensive on 26 October 1917, spearheaded by the Germans, and achieved a victory at Caporetto (Kobarid).

At the end of October, the Sinai and Palestine Campaign resumed, when General Edmund Allenby's XXth Corps, XXI Corps and Desert Mounted Corps (British Crops) won the Battle of Beersheba. Two Ottoman armies were defeated a few weeks later at the Battle of Mughar Ridge.

A Supreme War Council of Allied forces was created at the Doullens Conference on 5 November 1917.

The victory of the Central Powers at the Battle of Caporetto led the Allies to convene the Rapallo Conference at which they formed the Supreme War Council to coordinate planning. Previously, British and French armies had operated under separate commands.

Jerusalem was captured following another Ottoman defeat at the Battle of Jerusalem.

Fighting in Moldova continued in 1917, but Russian withdrawal from the war in late 1917 as a result of the October Revolution meant that Romania was forced to sign an armistice with the Central Powers on 9 December 1917.

In December, the Central Powers signed an armistice with Russia, thus freeing large numbers of German troops for use in the west. With German reinforcements and new American troops pouring in, the outcome was to be decided on the Western Front.

In January 1918, Romanian forces established control over Bessarabia as the Russian Army abandoned the province. Although a treaty was signed by the Romanian and Bolshevik Russian governments following talks between 5 and 9 March 1918 on the withdrawal of Romanian forces from Bessarabia within two months, on 27 March 1918 Romania formally attached Bessarabia, inhabited by a Romanian majority, to its territory, based on a resolution passed by the local assembly of that territory on its unification with Romania.

After this success, the number of Czechoslovak legionaries increased, as well as Czechoslovak military power. In the Battle of Bakhmach, the Legion defeated the Germans and forced them to make a truce.

In early 1918, the front line was extended and the Jordan Valley was occupied, following the First Transjordan and the Second Transjordan attacks by British Empire forces in March and April 1918.

German casualties between March and April 1918 were 270,000, including many highly trained stormtroopers.

Ludendorff drew up plans (code named Operation Michael) for the 1918 offensive on the Western Front. The Spring Offensive sought to divide the British and French forces with a series of feints and advances. The German leadership hoped to end the war before significant US forces arrived. The operation commenced on 21 March 1918 with an attack on British forces near Saint-Quentin. German forces achieved an unprecedented advance of 60 kilometers (37 mi).

Operation Georgette was the second large-scale German attack in spring 1918. The attack was smaller in scale than the original plan, so the codename was changed from “Georg” to “Georgette”. The offensive began at 4.15 am on 9 April 1918.

Romania officially made peace with the Central Powers by signing the Treaty of Bucharest on 7 May 1918. Under the treaty, Romania was obliged to end the war with the Central Powers and make small territorial concessions to Austria-Hungary, ceding control of some passes in the Carpathian Mountains, and to grant oil concessions to Germany. In exchange, the Central Powers recognised the sovereignty of Romania over Bessarabia.

During virtually continuous operations by Australian Light Horse, British mounted Yeomanry, Indian Lancers, and New Zealand Mounted Rifle brigades in the Jezreel Valley, they captured Nazareth, Afulah and Beisan, Jenin, along with Haifa on the Mediterranean coast and Daraa east of the Jordan River on the Hejaz railway. Samakh and Tiberias on the Sea of Galilee were captured on the way northwards to Damascus. Meanwhile, Chaytor's Force of Australian light horse, New Zealand mounted rifles, Indian, British West Indies and Jewish infantry captured the crossings of the Jordan River, Es Salt, Amman and at Ziza most of the Fourth Army (Ottoman Empire)

Germany launched Operation Marne (Second Battle of the Marne) on 15 July, in an attempt to encircle Reims. The resulting counter-attack, which started the Hundred Days Offensive, marked the first successful Allied offensive of the war.

By summer 1918, USA was sending 10,000 fresh soldiers to France every day.

The reorganized Egyptian Expeditionary Force, with an additional mounted division, broke Ottoman forces at the Battle of Megiddo in September 1918.

Serbian and French troops finally made a breakthrough in September 1918 in the Vardar Offensive, after most of the German and Austro-Hungarian troops had been withdrawn.

The battle was fought in the initial stage of the Vardar Offensive, in the Balkans Theatre. On 15 September, a combined force of Serbian, French and Greek troops attacked the Bulgarian-held trenches in Dobro Pole ("Good Field"), at the time part of the Kingdom of Serbia (present day Greece and North Macedonia). The offensive and the preceding artillery preparation had devastating effects on Bulgarian morale, eventually leading to mass desertions.

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 Austro-Hungarians failed to break through in a series of battles on the Piave and were finally decisively defeated in the Battle of Vittorio Veneto in October.

The Armistice of Mudros, signed at the end of October, ended hostilities with the Ottoman Empire when fighting was continuing north of Aleppo.

On 1 November, the Italian Navy destroyed much of the Austro-Hungarian fleet stationed in Pula, preventing it from being handed over to the new State of Slovenes, Croats and Serbs.

On 3 November, the Italians invaded Trieste from the sea. On the same day, the Armistice of Villa Giusti was signed.

By the end of hostilities in November 1918, Admiral Enrico Millo declared himself Italy's Governor of Dalmatia.

On 9 November 1918, having lost the support of the military, and with a revolution underway at home, Kaiser Wilhelm II was forced to abdicate his throne and flee Germany for Holland. Power was handed to a government led by the leader of the left-wing Social Democratic Party, Friedrich Ebert. The Treaty of Versailles

The Armistice of 11 November 1918 was the armistice signed at Le Francport near Compiègne that ended fighting on land, sea and air in World War I between the Allies and their last remaining opponent, Germany.

Austria-Hungary surrendered on 11 November 1918.

By mid-November 1918, the Italian military occupied the entire former Austrian Littoral and had seized control of the portion of Dalmatia that had been guaranteed to Italy by the London Pact.

The 1919 Paris Peace Conference imposed various settlements on the defeated powers, the best known being the Treaty of Versailles. The dissolution of the Russian, German, Ottoman and Austro-Hungarian empires led to numerous uprisings and the creation of independent states, including Poland, Czechoslovakia, and Yugoslavia. For reasons that are still debated, failure to manage the instability that resulted from this upheaval during the interwar period ended with the outbreak of World War II in 1939.

In the aftermath of the war, four empires disappeared: the German, Austro-Hungarian, Ottoman, and Russian. Numerous nations regained their former independence, and new ones were created.