During the four years of conflict, battle was joined by armies of unprecedented size, which were equipped with new mechanized technologies.

Other countries like Russia and the Ottoman Empire saw armies marching over much of their lands, with a great deal of resulting devastation.

Although the United States did join the war, due to Great Britain's control over the Atlantic Ocean, the only fighting for the U.S. Army was in Europe on the Western Front.

The war ended with Serbia and Greece taking the northern and southern halves of Macedonia and Thrace, respectively.

Serbia gained the region that is Kosovo in the modern day, Bulgaria was left a relatively small part of Thrace, and the Principality of Albania was formed on the western coast of the Balkans.

[2] The Battle of Kosovo is when Moravian Serbia was dissolved and Serbians were ruled by the Ottomans until the First Balkan War.

[3] During the trip, Ferdinand and his wife were traveling through the city in an open-topped car, when Gavrilo Princip, a Bosnian Serbian nationalist, fatally shot the archduke.

On 5 July 1914, German leader Kaiser Wilhelm II gave a "blank cheque" to Austria-Hungary, declared his country's allyship.

[7] It was sent on the 23rd, by which point French president Raymond Poincaré and his premier René Viviani had gone to Russia for a state visit, and would be unable to comment on the ultimatum in time.

[8] On 25 July, Serbia responded to the ultimatum, agreeing to most of its terms and offering the remaining disagreements to be subject to international arbitration.

On the 28th, Willhelm II—who had been on a cruise in the North Sea since 6 July—found out about the ultimatum to Serbia, and advised the German Foreign Office to tell Austria-Hungary to aim for an occupation of the Serbian capital of Belgrade rather than a full invasion.

The German Foreign Office had already told Austro-Hungarian monarch Franz Joseph I on the 27th that Germany authorized a full war.

On 2 August, German troops entered Luxembourg, and the next day, Germany declared war on France.

Britannica writes:[14] The outbreak of war in August 1914 was generally greeted with confidence and jubilation by the peoples of Europe, among whom it inspired a wave of patriotic feeling and celebration.

Few people imagined how long or how disastrous a war between the great nations of Europe could be, and most believed that their country’s side would be victorious within a matter of months.During the unstable European peace following Franco-Prussian War (1870—1871), the machine gun and the rapid-fire field artillery gun had been developed.

[14][15] At the start of the war, some major powers did not know that the development of such weapons ruined the effectiveness of frontal infantry or cavalry charges, and that trench warfare was a natural result of this.

The ship contained American passengers and its sinking contributed to the United States' later entry into the war on the side of the allies.

[27] The French passenger steamer SS Sussex was sunk by u-boats while crossing the British Channel on 24 March 1916; the sinking inspired the German military to take the Sussex pledge to not sink Allied ship containing passengers in certain areas, which was controversial among German military leaders.

Britannica writes: "The [Kerensky offensive] not only demonstrated the degree to which the Russian army had disintegrated, but also the extent of the Provisional Government’s failure to interpret and respond adequately to popular revolutionary sentiment.

[40][41] The Treaty of Brest Litovsk between the Russian SFSR and the Central Powers was signed in early 1918, making peace between the two and ending Russia's involvement in World War I.

[48] The Russian Civil War was won by the Red Army, and the Bolsheviks established the Soviet Union in 1922.