5 reasons that us joined world war 2.

1)Germans sinking US merchant ships taking supplies to England 
2)Japan's unreasonable demands on the United States (re- instating oil sales and dropping support of the Philipeenes and China)
3)German submarines beginning to shoot at US war ships 
4)England and France are typically our allies so we were going to enter the war eventually anyway 
5)Pearl Harbor was obviously why we entered the war.

Germany sank US merchant ships, Invasion in Japan, Japanese attack in pearl harbor England begged the US to be allies

Political forces often influence public opinion.