Perhaps it would be everybody's assumption that there are no other species other than humans that engage in wars. We mostly see ourselves as the monsters and think that animals kill in nature because it is the only way for them to feed themselves, and therefore survive. That is more than rational and acceptable.
However, there is a war between animals in history, which is also listed as 'Military Conflict' in Wikipedia: Gombe Chimpanzee War. This unusual war lasted for 4 years and was pretty bloody.
We, humans, aren't as unique as we think!