Ideas fight each other. The battles go on for centuries. The warriors are ordinary people led by a whole ranking of politicians, economists, religious leaders all pushing ideas they have obtained and often misinterpreted from prophets, philosophers and thinkers. Where one idea conflicts with another humans usually end up killing each other over the conflict.
Today in the world there are wars between those who believe in the concept of intolerance and those who believe in tolerance, wars between those who believe in greed at all costs and those who believe greed destroys all those it touches. A simple idea will often develop into a complicated one, leaving a bloody carnage in its wake, but, after all, there are plenty of humans in the world to kill in order to promote an idea.