WWI was caused by a number of things.
1. M. militarism. Prior to the war, European countries had a massive build up in military, competing with the others, in a sense. A. alliances. Nearly every country was an ally with another, which caused them to have to come to aid when the war broke out. I. imperialism. So many countries were pulled in because they imperialised and therefor had a whole arsenal of countries at their back. Imperialism also helped to develop tensions between countries. N. nationalism. Because so many people loved their country and would do anything for it.
What actually sparked the was was the assassination of the Duke Franz Ferdinand by the Black Hand, and M.A.I.N. pulled everyone in.
The US joined the war because of a number of things as well. The Zimmerman Note, where Germany asked Mexico to attack the US. And the attacking of vessels holding US citizens, who were killed when Germany attacked them, even though they were told not to as the US was neutral at the time.