What does Nature say about it?
Why is it so easy to switch into war mode for no particular purpose?

I've witnessed this in myself, much to my present chagrin. I could see the wrongness of Vietnam, and carried that value through the 80s and 90s. Then 9/11 knocked out the props, and I went along with everything Bush wanted for several years. It wasn't until 2006 that the Dubai Ports fiasco broke through the warfog. At that point I stopped thinking that Bush was "on the right side but communicating his aims poorly." I had to conclude that he was not working for America.

What does Nature say about pointless wars? And what do the old books say?

Nature is clear. Our close relatives shed plenty of blood in eating but not much in fighting. Birds and mammals often have disputes about territory and dominance but they rarely kill in those fights. They would much rather talk. Most fights are accomplished with visual, aural and olfactory symbols. Birds and mammals never venture into distant areas and start wars purely to create chaos. Termites are closer to humans in this respect.

The old books are much less clear.

The first part of the Old Testament is jampacked with Wilson/Bush/Obama-style wars for ideology. Jehovah wanted his people to defeat all other beliefs. But after the fall of the second temple in 70 AD, Jehovah's people switched from aggressive to defensive.

A similar division is recorded in the Koran. In the early parts, Mohammed was making war to spread his beliefs. In the later part, he's much more concerned with building a complete moral system in existing territories, with unbelievers subordinate but not dead.

Christianity's bipolar nature is more continuous. The scriptures don't start with territorial expansion; they were written in the same period when Judaism was turning defensive, and contain that spirit. But through the last 2000 years plenty of pointless wars have been fought nominally to spread Christianity. And right now all the pointless wars are started by nominal Christians who are working for nominal Jews.
  ¶ 3:33 AM