They are very vulnerable to physical damage but are going to fight a war with humans. An organization something like "star watchmen" try to make war look so bad that no one will fight one. The man selected to train the humans thinks war would be glamorous and really tries to train the humans. I think he finds out war is not glamorous after all.
The story features an alien race called Keligans who are a cylindrical species that resemble large caterpillers that are very vulnerable to damage. They are fighting in a war with humans in a controlled manner. There is reference to a multi-species police like group known as the Monitor Corps. I don't remember the story exactly myself, but I do remember that there is some mention of how war is not glamorous from the point of view of the human character Dermod.