The United States didn't join World War One until the final year in 1917. The US remained mostly neutral during the war in Europe and didn't have much to do with the actual causes. It wasn't until Germany violated the Sussex Pledge and issued the Zimmerman Note that the United States decided to join.
The beginning of the United States' consideration in joining was the sinking of the Lusitania by the Germans. The ship, carrying 1959 passengers, was sunk by a German U-boat on May 1, 1915. On the boat were some Americans, and this didn't sit well with President Woodrow Wilson. Despite this, he decided to remain neutral for two more years. The Lusitania was sunk by the Germans because it was carrying weaponry.
The main problem the Americans had with Germany was the unrestricted submarine warfare. Germany's U-boats were allowed to destroy practically any ship including merchant ones. Eventually this led to the sinking of the Sussex in 1916. The Sussex was a French steamer that was carrying Americans. Because of this, Wilson issued an ultimatum and eventually the Sussex Pledge which limited the German submarine warfare to not shoot down simply every ship in sight.
Despite this, Germany continued with the unrestricted warfare and thus the US was brought closer to the war. With the loss of American lives being unanswered for so long, something had to be done soon. However, hesitation existed due to the existence of the Russian empire which didn't agree with the ideas of democracy.
In 1917, two things happened that ultimately caused the United States to declare war on Germany. The first was the Russian Revolution. In this year the old Russian Empire was overthrown, and thus the United States felt greater comfort in aiding the Triple Entente. The second event was known as the Zimmerman Note. An encrypted telegram message was intercepted by the British that included an offer to Mexico by Germany to attack the United States. This note angered the United States in that Germany would have the audacity to cause the lives of more Americans.
And thus, the Americans joined the war.