Well in short the decline of the Ottoman empire left a huge power vacuum. A Serbian backed assassin killed ArchDuke Ferdinand of Austria/Hungary and his wife. Germany backed them thinking Great Britain would stay out of it. France, Russia(they entered for another reason), Belgium backed the Serbs and Great Britain joined along to Germany's shock. No one wants to back down as this was really a big territorial dispute to see who will be the next bully on the block and millions die.
US joins because they intercept a letter to Mexico from Germany that if they join and fight the US they get cash and parting gifts of Texas and New Mexico. Plus the US wasn't too keen on German subs sinking US passenger ships. In a stalemated war of attrition the US sends 1 million men in summer of 1918 and much needed supplies. 500,000 see action by the end of the war that November. Our entry provides a huge moral boost to the allies, the Germans now see a no win scenario. The basics of WW1
I know it is the short version but don't forget the influenza that infected half a billion people and killed between 50 and 100 million. Woodrow Wilson expanded the draft range to all men 18-45. Converted the nation into a war machine while ignoring the pandemic that was exploding as a result of his lust for war, all while taking a giant dump on the rights of Americans. The reason it is called the Spanish flu was because the freedom hating leaders of every other country including ours where it most likely started silenced the press. Spain was the only nation that was reporting on it.