In June 1914 the world was ruled by great empires
, and the ideals of nationalism
ran wild on the streets of Europe
. But these ideals were strongest in Serbia's desire to create a united south-Slav state. In our timeline these ideals reached a boiling point and finally one of the brutal wars of all time broke out. The war was fought by the Triple Alliance (Central Powers)
and the Triple Entente (Allied Powers)
between 1914 and 1918. With the war ending in 1918 resulting in the defeat of the Central Powers and the Treaty of Versailles. Resulting in bitterness and hatred and 20 years later a second world war that was far more terrible than the first.
The main shift in the timeline I believe to be at the Pless conference in 1917. It was this conference between the German military, civilian government and Kaiser Wilhelm II that in our timeline saw the resumption on unrestricted submarine warfare. Due to his earlier commitment to a policy of moderation the Kaiser, uncharacteristically, sides with Bethmann-Hollweg who opposed the Navy's plan. This does not result in any immediate changes to the war, it is pivitol in keeping the United States neutral. Without American support in Europe and the collapse of the Russian Empire, Allied morale would suffer a fatal blow.
The Germans would inflict more damage on the Allied armies in the west with their 1918 Spring Offensive. While never capturing Paris, the French government evacuates and replaces Georges Clemenceau with the more conciliatory Aristide Briand. Calls by Ferdinand Foch for a counter-attack would be denied because of the government would fear another mutiny in the French Army. Briand would consult with the other Allies over the summer to determine if they'd agree to an armistice with the Central Powers. The United Kingdom would agree due an ongoing crisis in Ireland over conscription. Italy, still recovering from Caporetto, agreed when it became clear that without support from it's allies offensive action by Italy with success assured would be impossible.
With the major Entente Powers in agreement, the smaller powers fell in line. An armistice
between all the belligerent nations on 6 September 1918. The Lausanne Peace Conference
results in a treaty
that officially ends the war in 1919.