Answer:
I could be wrong but I am pretty sure the answer is Oil.
Explanation:
After America cut off oil from Japan, due to expansion reasons I believe, Japan got really upset and Pearl Harbor happened if I remember correctly! Hope this helped!! :)
The answer is Great Depression
These people became wealthy mostly because of efficient trade between countries. Trading was an important factor back then, and your key word is COASTAL, meaning they are by a sea (aka trading routes). So the answer is trading between countries.