sources

https://www.google.com/search?q=world+war+2&rlz=1C1GGRV_enUS758US758&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiy1qfom_jWAhVm7oMKHUDPB0wQ_AUICigB&biw=1366&bih=637#imgrc=4-_6sFTImCbUsM:

https://owlcation.com/humanities/Reasons-for-American-Entry-Into-WWII

http://historylists.org/events/5-reasons-for-the-us-entry-into-world-war-i.html