The British had a blockade on German ports, which made Germany trading with U.S impossible. The U.S still traded with France Britain anyways, which angered Germany.
The Germans torpedoed a British passenger ship called the Lusitania with a submarine. They killed 1,000 passengers, including 128 Americans.
After the sinking of the Lusitania, Germany pledged that its submarines would warn the people on a ship that they were going to sink the ship before they actually torpedoed a ship. This was called the Sussex Pledge.
We're gonna sink your ship!
In January 1917, Germany announced that it would sink all of the ships sailing into allied ports. Which totally went against the Sussex Pledge.
We're gonna sink that ship!
The British intercepted a telegram that was sent from Germany to Mexico. The telegram was the Germans trying to get Mexico to invade the U.S if the U.S joined the war in Europe. It was known as the Zimmerman note. This angered many Americans.