World War I (often abbreviated as WWI or WW1), also known as the First World War or the Great War, was a global war originating in Europe that lasted from 28 July 1914 to 11 November 1918. Contemporaneously described as "the war to end all wars",[7] it led to the mobilisation of more than 70 million military personnel, including 60 million Europeans, making it one of the largest wars in history.[8][9] It is also one of the deadliest conflicts in history,[10] with an estimated nine million combatants and seven million civilian deaths as a direct result of the war, while resulting genocides and the resulting 1918 influenza pandemic caused another 50 to 100 million deaths worldwide.[11]

Gavrilo Princip (Serbian Cyrillic: Гаврило Принцип, pronounced [ɡǎʋrilo prǐntsip]; 25 July 1894 – 28 April 1918) was a Bosnian Serb member of Young Bosnia who sought an end to Austro-Hungarian rule in Bosnia and Herzegovina. At the age of 19, he assassinated Archduke Franz Ferdinand of Austria and the Archduke's wife, Sophie, Duchess of Hohenberg, in Sarajevo on 28 June 1914. Princip and his accomplices were arrested and implicated as a nationalist secret society, which initiated the July Crisis and led to the outbreak of World War I.

Archduke Franz Ferdinand Carl Ludwig Joseph Maria of Austria (18 December 1863 – 28 June 1914) was the heir presumptive to the throne of Austria-Hungary.[1] His assassination in Sarajevo is considered the most immediate cause of World War I. Franz Ferdinand was the eldest son of Archduke Karl Ludwig of Austria, the younger brother of Emperor Franz Joseph I. Following the suicide of Crown Prince Rudolf in 1889 and the death of Karl Ludwig in 1896, Franz Ferdinand became the heir presumptive to the Austro-Hungarian throne.

The July Crisis was a series of interrelated diplomatic and military escalations among the major powers of Europe in the summer of 1914 that was the ultimate cause of World War I. The crisis began on June 28, 1914, when Gavrilo Princip, a Bosnian Serb, assassinated Archduke Franz Ferdinand, heir presumptive to the Austro-Hungarian throne. A complex web of alliances, coupled with miscalculations by many leaders that war was in their best interests or that a general war would not occur, resulted in a general outbreak of hostilities among almost every major European nation in early August 1914; every major European nation was involved by May 1915.

A great power is a sovereign state that is recognized as having the ability and expertise to exert its influence on a global scale. Great powers characteristically possess military and economic strength, as well as diplomatic and soft power influence, which may cause middle or small powers to consider the great powers' opinions before taking actions of their own. International relations theorists have posited that great power status can be characterized into power capabilities, spatial aspects, and status dimensions.[4]

The Triple Alliance was an agreement between Germany, Austria-Hungary, and Italy. It was formed on 20 May 1882[1] and renewed periodically until it expired in 1915 during World War I. Germany and Austria-Hungary had been closely allied since 1879. Italy was looking for support against France shortly after it lost North African ambitions to the French. Each member promised mutual support in the event of an attack by any other great power. The treaty provided that Germany and Austria-Hungary were to assist Italy if it was attacked by France without provocation. In turn, Italy would assist Germany if attacked by France. In the event of a war between Austria-Hungary and Russia, Italy promised to remain neutral. The existence and membership of the treaty were well known, but its exact provisions were kept secret until 1919.

The Schlieffen Plan (German: Schlieffen-Plan, pronounced [ʃliːfən plaːn]) was a name given, after the First World War, to German war plans, the influence of Field Marshal Alfred von Schlieffen and his thinking on an invasion of France and Belgium which began on 4 August 1914. Schlieffen was Chief of the General Staff of the German Army from 1891 to 1906. In 1905 and 1906, Schlieffen devised an army deployment plan for a war-winning offensive against the French Third Republic. After losing the First World War, German official historians of the Reichsarchiv and other writers described the plan as a blueprint for victory.

The German invasion of Belgium was a military campaign which began on 4 August 1914. Earlier, on 24 July, the Belgian government had announced that if war came it would uphold its historic neutrality. The Belgian government mobilised its armed forces on 31 July and a state of heightened alert (Kriegsgefahr) was proclaimed in Germany. On 2 August, the German government sent an ultimatum to Belgium, demanding passage through the country, and German forces invaded Luxembourg.

The Treaty of London of 1839, also called the First Treaty of London, the Convention of 1839, the Treaty of Separation, the Quintuple Treaty of 1839, or the Treaty of the XXIV articles, was a treaty signed on 19 April 1839 between the Concert of Europe, the United Kingdom of the Netherlands and the Kingdom of Belgium. It was a direct follow-up to the 1831 Treaty of the XVIII Articles which the Netherlands had refused to sign, and the result of negotiations at the London Conference of 1838–1839.[1]

The Central Powers, also Central Empires[1] (German: Mittelmächte; Hungarian: Központi hatalmak; Turkish: İttifak Devletleri / Bağlaşma Devletleri; Bulgarian: Централни сили, romanized: Tsentralni sili), consisting of Germany, Austria-Hungary, the Ottoman Empire and Bulgaria - hence also known as the Quadruple Alliance[2] (German: Vierbund)—was one of the two main coalitions that fought World War I (1914–18).

The First Battle of the Marne (French: Première bataille de la Marne, also known as the Miracle of the Marne, Le Miracle de la Marne) was a World War I battle fought from 6–12 September 1914.[1] It resulted in an Allied victory against the German armies in the west. The battle was the culmination of the German advance into France and pursuit of the Allied armies which followed the Battle of the Frontiers in August and had reached the eastern outskirts of Paris. A counter-attack by six French armies and the British Expeditionary Force (BEF) along the Marne River forced the Imperial German Army to retreat northwest, leading to the First Battle of the Aisne and the Race to the Sea. The battle was a victory for the Allied Powers but led to four years of trench warfare stalemate on the Western Front.

Attrition warfare is a military strategy consisting of belligerent attempts to win a war by wearing down the enemy to the point of collapse through continuous losses in personnel and materiel. The war will usually be won by the side with greater such resources.[1] The word attrition comes from the Latin root atterere to rub against, similar to the "grinding down" of the opponent's forces in attrition warfare.[2]

The Eastern Front or Eastern Theater of World War I (German: Ostfront, Russian: Восточный фронт, Vostochny front) was a theater of operations that encompassed at its greatest extent the entire frontier between the Russian Empire and Romania on one side and the Austro-Hungarian Empire, Bulgaria, the Ottoman Empire and the German Empire on the other. It stretched from the Baltic Sea in the north to the Black Sea in the south, involved most of Eastern Europe and stretched deep into Central Europe as well. The term contrasts with "Western Front", which was being fought in Belgium and France.

The Zimmermann Telegram (or Zimmermann Note or Zimmerman Cable) was a secret diplomatic communication issued from the German Foreign Office in January 1917 that proposed a military alliance between Germany and Mexico. If the United States entered World War I against Germany, Mexico would recover Texas, Arizona, and New Mexico. The telegram was intercepted and decoded by British intelligence.

The American Expeditionary Forces (A. E. F., A.E.F. or AEF) was a formation of the United States Army on the Western Front of World War I. The AEF was established on July 5, 1917, in France under the command of Gen. John J. Pershing. It fought alongside French Army, British Army, Canadian Army, and Australian Army units against the Imperial German Army. A minority of the AEF troops also fought alongside Italian Army units in that same year against the Austro-Hungarian Army.

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 opponent, Germany. Previous armistices had been agreed with Bulgaria, the Ottoman Empire and the Austro-Hungarian Empire. Also known as the Armistice of Compiègne from the place where it was signed at 5:45 a.m. by the French Marshal Foch,[1] it came into force at 11:00 a.m. Paris time on 11 November 1918 and marked a victory for the Allies and a defeat for Germany, although not formally a surrender.

The Big Four or the Four Nations refer to the four top Allied powers of the World War I[1] and their leaders who met at the Paris Peace Conference in January 1919. The Big Four is also known as the Council of Four. It was composed of Woodrow Wilson of the United States, David Lloyd George of the United Kingdom, Vittorio Emanuele Orlando of Italy, and Georges Clemenceau of France.[2]

The League of Nations, abbreviated as LN or LoN, (French: Société des Nations [sɔsjete de nɑsjɔ̃], abbreviated as "SDN" or "SdN" and meaning "Society of Nations") was the first worldwide intergovernmental organisation whose principal mission was to maintain world peace.[1] It was founded on 10 January 1920 following the Paris Peace Conference that ended the First World War; in 1919 US President Woodrow Wilson won the Nobel Peace Prize for his role as the leading architect of the League.