Nations go to war to gain access to new lands, to gain access to economic resources and to defend themselves against perceived threats from other entities. Nations throughout history have gone to war based on differences in religious, cultural and political beliefs, and values.
In some cases, populations mired in territories with limited natural resources feel compelled to fight for new lands for survival purposes. In other cases, it is economic expansion or greed that cause nations to go to war to take over other countries or lands. Often, the wars centered on religious or philosophical beliefs are a fight for control over a way of life in an area.