What caused World War 1?
Two major causes of the US getting involved in WWI are
1) German use of "unrestricted submarine warfare." German submarines sank any ships (no matter what country they were from or if they had passengers) headed for Britain. This was against international law and some Americans got killed this way. This angered the Americans. The most famous example of this is the ship "Lusitania."
2) Zimmermann Telegram. The Germans tried to get Mexico to declare war on the US. They promised that if Mexico helped, Germany would give the Southwest (that we took from Mexico in the 1840s) back to Mexico after they won the war.