Before the United States entered World War I, they tried their best to stay neutral. President Woodrow Wilson was a huge believer in peace and he did not want his people to have to go through war. He wanted the best for his people and did not want the burden of a war, so he tried his best to stay out of it.
The sinking of the Lusitania occurred in 1915. This was where a ship got sunk with Americans on board, which caused outrage among the United States. People were devastated and many wanted President Woodrow Wilson to declare war and finally join World War I. Even though he was angered by the sinking of the Lusitania, he still tried his best to stay neutral and decided not to join the war.
In 1917, Great Britain had obstructed a telegram, called the Zimmermann Telegram, from Germany to Mexico. Within this telegram, it stated that if Mexico were to attack the United States, if Germany won the war they would give American land to Mexico. This was very threatening towards the United States, as the United States was considered the most dominant power within North and South America. They also did not want to shed blood on American territory.
Eventually, this caused President Woodrow Wilson to declare war. The Zimmermann Telegram was too threatening to ignore and Wilson wanted to protect his people. This caused the United States to join World War I on April 6, 1917.