War is a time of conflict with intent to harm or kill, supported by the opposing countries or societies. Since there have been records, humanity has desired peace, namely the absence of war.
War can be for any reason, these are the most common:
One nation seeks to take advantage of another nation’s natural resources
Ownership and control, particularly of land/territory
Religious or belief, and the view that unbelievers should be disenfranchised or killed. This includes genocide or any form of violence that is based on the group is not entitled to basic rights of life or liberty. (See Genocide below)