it started out as a war between Serbia and Austria-Hungary. But quickly became much more. The original allies at the beginning of the war were France, Britain, and Russia. They were known as the Triple Entente. Where as Germany, Austria-Hungary, the Ottoman Empire, and Bulgaria were known as the Quadruple Alliance. Italy originally had an alliance with Germany but left when they started invading other countries.
The United States's president at the time was Woodrow Wilson. He asked to congress to declare war on Germany shortly after being elected mainly on his slogan "He kept us out of war." Although he didn't stick to his slogan, many Americans agreed with his decision to go to war. The U.S joined the war because of a telegram intended for Mexico from Germany was intercepted by Britain. The telegram said that if Mexico attacked the U.S and kept them out of the war in Europe then Germany would help Mexico take back the border states.
42 million troops were deployed on the allies side. Around 5 million of those troops died. More than 4,350,000 american troops were in the war but only 53,402 of those soldiers died. The Allies did much better compared to the Central powers, who had 25 million and lost 16 million. Both sides took heavy causalities and there were big loses for both sides
The cost of the war for the countries involved was tremendous. It was one of the most expensive wars in history. The war cost the allies $125,690,477,000 to fight this war compared to the Central powers $60,643,160,000. The Allies spent a little more than double the Central powers did. Although the Allies did have a lot more nations than the Central powers did.