So, the World War 1 started with the assassination of Franz Ferdinand which led to a series of military conflicts between several different nations in Europe.
However, one may ask what was America’s interest in joining the WW1 especially since it wasn’t a European nation and had nothing to do with the European conflict. For the first 3 years USA decided to stay out of the war which was a wise decision.
President Woodrow Wilson wanted to have political control so that he could be reelected as the POTUS again. He wanted the German Americans and the Irish Americans who supported the Central Powers to remain in his favor and thus, he initially declared that the US would never enter the war.
However, after being reelected in late 1916, the US went to war in 1917 just 6 months after he was reelected. There were several reasons for USA entering the conflict. The major chunk of citizens in the USA were beginning to dislike Germany.
Germany incited Mexico to form an alliance against the USA via the Zimmermann telegram. This telegram was the major reason why the USA entered the war against Germany.
Germany was gaining force and could become an autocratic nation difficult to deal with in the near future. Also, USA was gaining huge business from exports to the Allied Forces – Britain, France and Italy. The unrestricted submarine warfare angered the US a lot as Germany was causing a loss of revenue to the USA by attacking their merchant ships.
USA had also lent loans to the Allied Forces. Hence, if the Allied Forces were conquered the loans lent to them would not be obtained back.
So, because of these reasons USA decided to enter the WW1.