The Allied forces were, The United States, The United Kingdom and France. The Central Powers were, Germany, Austria-Hungary and Italy. It involved over 70 million military personnel, including 60 million Europeans.
All this manpower, and equipment, was mobilized for what was to be the sixth-deadliest conflict in world history. It was a deadly war that resulted in the deaths of over 9 million combatants.
The conflict was started when Austria-Hungary invaded Serbia. Around the same time the Germans invaded Belgium, Luxembourg and France. It also included a Russian attack against Germany.
Once the Germans were halted on their march towards Paris, the war became a series of indecisive battles, along the Western Front. that changed very little until 1917. It was a long war fought by men in trenches. According to a PBS documentary called, The Trenches: Symbol of the Stalemate, 12,000 miles of trenches were dug by both sides during the war.
World War I was called the "Great War" and the war to end all wars. However, we know there has been one other world war since WWI ended. There have been numerous other conflicts since the armistice that brought a halt to the war. Most know that WWI was centered in Europe and involved all the world's great powers.
The history books tell us that The United States never formally became a member of the Allied powers, but rather they were what was called a self-styled "Associated Power.” Our nation had a small army until after the Selective Service Act was passed. That act allowed the drafting of 2.8 million men. It’s said by the summer of 1918, we were sending 10,000 new soldiers to France every day. WWI was the first war that involved fixed-wing aircraft on a large scale.