I'm not sure a military man will give peace a chance when needed...
"Let him who desires peace, prepare for war." -Flavius Vegetius, 375 AD
I look at the above quote, because I think your idea of what a military man is may not be correct. The most important part of a military force is preparation. War can be fought as an invasion force --but it can also be fought defensively. The military isn't about desiring war --it is about preparing for war in case it is necessary. (I'm not going to open the can of worms about what is and isn't necessary --everyone's got an opinion on this one, and even when they've got one, it can be subject to change. I know mine has over the years).
If anything, I think a military man is more
likely to give peace a chance. Especially if he has combat experience. Good generals don't like sending people to die. Good commanding officers shudder at the thought of writing letters to a parent/spouse/etc. that their family member has been killed in action. I think that once you've seen death, the desire to avoid unneccessary death becomes that much more earnest.
However, that same military man knows best how to prepare when all diplomatic means are exhausted, or if it becomes clear that an invasion is inevitable. I think that person is more likely to consider what needs to be done to prevent losing more lives than needed, as well as to make sure a military campaign is fully committed and successful (i.e., if we really do
need to go somewhere, how to do it and get it done, rather than do an LBJ in Vietnam), and to not underestimate the job at hand. He'd also be the one to ensure that members of the military have the equipment they need to get this job done effectively, and with the least amount of risk.
Finally, as Ian said, it is one cabinet post out of an entire cabinet. I'm not advocating a cabinet of military personnel, but one member who has perspective that other members may not.