If you use Google Trends (a tool to illustrate the frequency of search terms) and search for the word "war" with USA as geographic region, you will be presented with the graph below showing monthly data from 2004 - 2019 (I have added the dates for emphasis);
However, if you search for the word "peace," there is no such periodicity;
If you change the region to another English speaking country such as the United Kingdom, the periodicity of "war" is still somewhat present but less pronounced, appears to be decaying and is instead shifted to peak around September - November.
What would be an efficient way to determine the cause of the periodicity of "war" (search region USA)?
Since correlation does not imply causation, I don't know how to effectively tackle this problem mathematically. Any ideas/approaches would be much appreciated!
EDIT: As has been suggested in the answer/comments below there may be a simple connection with US holidays. For example, searching for "war,memorial day" yields the following graph;