Note also that you should limit the subject. Trying to address all the causes of war would take far more space than the 5 pages you do have. As this in your last major paper, show me that you understand the basics of college writing. I expect a title that works, and introduction that catches my attention, a clear thesis that indicates why you are speculating about the causes, valid arguments, quotes that are incorporated into the paper, and a conclusion that ends forcibly. A trend is simply a change that occurs over a long period of time.
Fads are temporary, mostly insignificant changes in preference or action.
A phenomena is best considered to be a fairly permanent pattern or activity. Older students attending college is a phenomena. The sinking of the Titanic is an event. In order to ensure a bit of thought and critical thinking, you are not to write on a fad or an event. Choosing a trend that is no longer a trend.