World War I

World War I was a global conflict that began in Europe and lasted until November 11, 1918. 

The Great War, also known as "the war to end all wars," resulted in the mobilization of almost 70 million armed troops, including 60 million Europeans, making it one of the most massive wars in history. 

It was also one of the deadliest conflicts in history, with an estimated 8.5 million combatants and 13 million civilians killed directly as a result of the war, while genocides and the 1918 Spanish flu pandemic killed another 17–100 million people worldwide, including 2.64 million Spanish flu victims.

Gavrilo Princip, a Bosnian Serb Yugoslav nationalist and member of the Serbian Black Hand military organization, assassinated Archduke Franz Ferdinand of Austria-Hungary in Sarajevo on June 28, 1914, sparking the July Crisis.  In response, on July 23, Austria-Hungary issued an ultimatum to Serbia. The Austrians were not satisfied with Serbia's response, and the two countries went to war. 

The crisis grew from a bilateral issue in the Balkans to one encompassing most of Europe as a result of a web of interlocking alliances. By July 1914, Europe's main powers had split into two coalitions: the Triple Entente, which included France, Russia, and the United Kingdom; and the Triple Alliance, which included Germany, Austria-Hungary, and Italy. 

The Triple Alliance remained purely defensive, allowing Italy to stay out of the war until April 26, 1915, when ties with Austria-Hungary worsened and it joined the Allied Powers. After Austria-Hungary attacked the Serbian capital of Belgrade, which was only a few kilometers from the border, on July 28, 1914, Russia believed it was imperative to support Serbia and ordered partial mobilization.

On the evening of July 30, Russia announced full mobilization; the next day, Austria-Hungary and Germany did the same, with Germany demanding that Russia demobilize within twelve hours.  

When Russia refused, Germany declared war on Russia on August 1, 1914, in support of Austria-Hungary, which did so on August 6, 1914. On August 2, 1914, France ordered a complete mobilization in support of Russia.   

In the end, World War I split Europe into two major opposing alliances: the Allied Powers, which included the United Kingdom of Great Britain and Ireland, the United States, France, the Russian Empire, Italy, Japan, and Portugal. Also, it many of the Balkan States such as Serbia and Montenegro as well as the Central Powers which was mainly made up of the German Empire, the Austro-Hungarian Empire, the Ottoman Empire and Bulgaria.

Germany's strategic approach for a two-front conflict against France and Russia was to focus the bulk of its military in the West for six weeks to annihilate France, then transfer troops to the East before Russia could completely mobilize, which became established as the Schlieffen Plan.  Germany started demanding free passage through Belgium on August 2nd, a requirement for a rapid war with France.  

When it was denied, German forces entered Belgium on the 3rd of August and declared war on France the same day. The Belgian government exercised the 1839 Treaty of London, and Britain declared war on Germany on the 4th of August, in accordance with its commitments under the treaty.

On 12 August, Britain and France also declared war on Austria-Hungary; on 23 August, Japan sided with Britain, seizing German possessions in China and the Pacific. In November 1914, the Ottoman Empire entered the war on the side of Austria-Hungary and Germany, opening fronts in the Caucasus, Mesopotamia, and the Sinai Peninsula. 

The war was fought in (and drew upon) each power's colonial empire also, spreading the conflict to Africa and across the globe.

Britain and France declared war on Austria-Hungary on August 12th, and Japan sided with Britain on August 23rd, seizing German assets in China and the Pacific. The Ottoman Empire joined the war in November 1914, siding with Austria-Hungary and Germany and establishing fronts in the Caucasus, Mesopotamia, and the Sinai Peninsula. 

The war was also fought in each power's colonial empires, spreading the conflict throughout Africa and the world.

The Battle of the Marne stalled the German advance into France, and by the end of 1914, the Western Front had settled into a bloody war, marked by a long series of trench lines that didn't change much until 1917.  

Italy joined the Allies in 1915 and established a front in the Alps. Bulgaria joined the Central Powers in 1915, and Greece joined the Allies in 1917, broadening the scope of the conflict with in Balkans. The United States initially stayed neutral, but it became an efficient producer of war materiel to the Allies while still neutral.

Following the sinking of American merchant ships by German submarines, Germany's statement that its navy would continue unrestrained assaults on neutral shipping, and evidence that Germany was attempting to spark Mexico to declare war on the United States, the United States declared war on Germany on April 6, 1917. 

Although trained American forces did not begin arriving in large numbers at the front until mid-1918, the American Expeditionary Force eventually grew to over two million troops.

Despite the fact that Serbia was defeated in 1915 and Romania joined the Allied Powers in 1916 only to be defeated in 1917, none of the major powers were defeated until 1918. 

The Provisional Government replaced the Monarchy in Russia after the February Revolution in 1917, but continued dissatisfaction with the war's cost led to the October Revolution, the formation of the Soviet Socialist Republic, and the new government's signing of the Treaty of Brest-Litovsk in March 1918, effectively ending Russia's involvement in the war. 

Germany had gained control of much of eastern Europe and had dispatched a large number of additional troops to the Western Front.

The German March 1918 Offensive was initially successful, thanks to new tactics. The Allies retreated and held their ground. As 10,000 new American troops arrived every day, the last of the German reserves were depleted. In their Hundred Days Offensive, the Allies pushed the Germans back with a series of attacks against which the Germans had no countermove.  

The Central Powers left one by one: Bulgaria in September 29, the Ottoman Empire October 31, and the Austro-Hungarian Empire November 1.  With its allies defeated, a revolution at home, and a military unwilling to fight, Kaiser Wilhelm abdicated on November 9th, 1918, and Germany signed a cease fire on November 11th, 1918, bringing the war to a close.

World War I marked a watershed moment in the world's political, cultural, economic, and social climates. Several revolutions and uprisings erupted as a result of the war and its subsequent collapse. 

The defeated powers were forced to accept the terms of the Big Four in a series of treaties negotiated at the 1919 Paris Peace Conference, the most famous of which was the Treaty of Versailles with Germany. 

The Austro-Hungarian, German, Ottoman, and Russian Empires all ceased to exist as a result of the war, and numerous new states arose from their ashes. Despite the overwhelming Allied victory, a second world war broke out just over two decades later.