On June 28, 1914, Gavrilo Princip, an agent of the Serbian Black Hand secret society, assasinated Archduke Franz Ferdinand, heir to the Austria-Hungary throne. This set the wheels turning that started the first World War. Along with revolutionary unrest in many European countries.
The same day of Archduke Franz Ferdinand's assassination the Austrian government retaliated by declaring war on Serbia. Russia's Emperor, Nicholas II was pressured by Russia's military to defend Serbia. So after much regrets he declared war on Austria-Hungary. Germany, being power hungry, and allied with Austria-Hungary, then declared war on Russia. Then, following its alliance with Russia, France declared war on Germany. And that was the first of the complicated beginnings of World War I.
Later Britain followed its "Triple Entente" with France and Russia by declaring war on Germany. But the United States kept a Neutral state and tried to keep trade up with both Britain and Germany, but by 1916 Britain had been able to cut off 99% of American trade with Germany. So Germany gave unbridled power to its submarines by allowing them to attack all merchant ships headed to England. American President Woodrow Wilson kept neutrality even after the sinking of some of American ships. But after the an intercepted telegram from Germany to Mexico, proposing an alliance against the United States, the president asked Congress for a Declaration of War on April 2, 1916, and got it. The American government campaigned to tell the public that entering the war was good. In the end, America sent nearly 4 million men to Europe.
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In March of 1918 Russia, undergoing revolutionary movements, made a separate peace with Germany and left the war. Germany and the Central Powers turned their attention to the West and the New American threat. With an almost inexhaustible supply of manpower, America's inexperienced "doughboys", with the help of Allied Forces slowly pushed German forces back.