It was a war, and it involved pretty much every major power in the world. The major world powers wanted to protect their trade interests. They stockpiled their armies. Two major alliances were formed: Triple Alliance (Germany, Austria- Hungary, Italy,) and Triple Entente (France, Russia, Britain)
It started because all the powers wanted land. Most of them were enemies to begin with. Russia and Austria- Hungary wanted the Balkans, so they started fighting. Germany defended Austria- Hungary. Then they declared war on France.
The one event that started the war was when Archduke Franz Ferdinand (future emperor of Austria- Hungary) was assassinated by a Bosnian guy. Germany invaded France. They failed.
Two sides were formed. The Central Powers (Germany, Austria- Hungary, Ottoman Empire) and the Allies (Britain, France, Russia, Greece, Japan, Italy). The US remained neutral, but the Americans were closer with the Allies.
Germany continued to attack people. They broke all the rules of war. They used a new weapon (a submarine) and they sunk a British passenger ship called the Lusitania. 128 Americans died out of the total of 1198 killed. The American people were really angry with Germany.
The US warned Germany not to sink any more passenger ships, but they did it again when they sank the French passenger ferry. No Americans were on board but Wilson still warned Germany about targetting unrestricted submarine warefare. in 1916, The Germans signed something called the Sussex pedge:
Germany broke the Sussex pledge in 1917. Also, a letter was intercepted that was from Germany to Mexico. The note ("Zimmerman Note") talked of invading America
April 4, 1917: President Wilson declares war.
Lots of American men joined the army. Bonds were sold to the public to raise money. Acts were passed that sent anyone talking bad about the US govt. to jail. Americans joined the Allies and fought the Central powers. The US sent large convoys over and helped defeat the Central powers.
November 11, 1918: A cease-fireis agreed to on what is now known as Armistice day. The Allies won.
President Wilson wanted peace. He came up with a plan that had 14 points for peace. It had many points about countries not fighting and stuff. The major point was the formation of a League of Nations (like the League of Justice except with countries instead of superheroes.)
Wilson went to France and the Allied powers signed the Treaty of Versailles (1919). It had Wilson’s 14 points.
One of the rules from the treaty was that Germany had to pay the Allies $56 billion for war damages. Wilson blamed Germany for starting the war.
The US Senate didn’t like the treaty or Wilson’s 14 points. They rejected both because they didn’t want to become part of the League of Nations.