Here is a cool report from National Geographic:
Do other animals go to war? Since our close relative the chimpanzee is often thought of as war-like, we took the question to Nicholas E. Newton-Fisher, a primate behavioral ecologist at the University of Kent.
“It probably depends on the definition of ‘war,'” Newton-Fisher says by email. “There’s good evidence that chimpanzees conduct deliberate raids on neighbouring communities, and that this can lead to annexing of territory."
For instance, during a ten-year study of a chimp family in Uganda's Kibale National Park, the primates killed or injured 18 chimps from other groups and took over their land. (Related: "Chimp Gangs Kill to Expand Territory.")
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The author seems to be all tied up in semantics. What is the definition of war?
: a state or period of fighting between countries or groups
: a situation in which people or groups compete with or fight against each other
: an organized effort by a government or other large organization to stop or defeat something that is viewed as dangerous or bad
Source - http://www.merriam-webster.com/dictionary/war
Take away the nation state factor and the human bias, and it seems clear to me that other animals do indeed go to war. And since it's mainly "hand-to-hand combat", their wars are just as brutal as ours.