Before WW1, a lot of countries in Europe wanted to have more control over the land and they wanted to their race and civilizations to be noticed. When one nation started using violence, Europe would go into war.
Franz Ferdinand was the Archduke of Austria. It was said that his assassination was the main event that started WW1 since it was the turning point which countries needed in order to start fightong.
Militarism, alliances, imperialism, and nationalism were the main and immediate causes of WW1. All of the European countries were fighting to achieve this, therefore, the war started.
The United States decided to enter the war since they had a lot of economic power. They wanted to help by supplying munitions and food to England.
There was this boat in Germany that sunk in 1915. There were 128 US Citizens that were killed. The US was really angry therefore they decided that the best way that they could revenge would be joining the war.
The German Prime Minister sent a telegram to Mexico asking for its union in the war in order to fight the United States. Mexico declined and since the US found out they decided to fight the war against Germany.