The rise of nationalism, or a strong attachment to one's homeland, was one such development. Within a country, nationalism can function as a unifying force.
The Ottoman Empire, which included the Balkans, was rapidly fading by the early 1900s.
The heir to the Austro-Hungarian throne, Archduke Franz Ferdinand, and his wife, Sophie, plunged into this poisoned atmosphere of mutual hate and suspicion.
The pair paid a state visit to Sarajevo, Bosnia's capital, on June 28, 1914. It would be their final meeting.
The combat lines were firmly drawn by mid-August 1914.
On one hand, there was Germany, and on the other, there was Austria-Hungary.
On the Western Front, opposing troops had constructed kilometres of parallel trenches to protect themselves from enemy fire by early 1915.
Russia's war effort was on the verge of collapsing by 1916. Russia, unlike the countries of Western Europe, had not yet been industrialized.
An attack on the Dardanelles, a province of the Ottoman Empire, appeared to be a promising option for the Allies.
That year, the Germans ramped up submarine warfare in the Atlantic Ocean, which had been raging since the war began.
Civil unrest in Russia pushed Czar Nicholas to resign in March 1917, owing in large part to war-related food and fuel shortages.
Due to Russia's departure from the war, Germany was finally able to deploy practically all of its soldiers to the Western Front. The Germans launched one final, overwhelming offensive on the Allies in France in March 1918.
President Wilson drafted a series of peace proposals in January 1918, while the war was still raging.
As the Paris Peace Conference began, neither the United Kingdom nor France showed any signs of agreeing with Wilson's concept of peace.
In 1919 and 1920, the Western powers negotiated separate peace treaties with each of the vanquished countries: Austria-Hungary, Bulgaria, and the Ottoman Empire.
The Treaty of Versailles, in the end, achieved little to establish a permanent peace. For one instance, the pact was ultimately rejected by the United States, which was considered the world's preeminent power after the war.
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