In June 1914 the world was ruled by great empires, and the ideals of nationalism and democracy ran wild on the streets of Europe and Russia. 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 greatest 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 being enforced on the losers. Resulting in bitterness and hatred and 20 years later a second world war that was far more terrible than the first.
The Central Powers had key opportunities to win world war I. If coordinated properly the German Navy could have broken the British blockade, Mexico may have been able to hold out if they entered the war on the side of the Central Powers, occupied eastern Europe could have sent men to fight as reinforcements or reserves. The revolts in Ireland could have caused the British to pull out. These events shaped the tide of the war and it would be at these times in the war the the Central Powers would be decisive and victorious.
The river of time continues to flow to the present day. But because of these altered events the course has shifted.