Humanity has shown that war is possible under any circumstances.
This has anything to do with anything because...? Over distances of time and space (ftl is still not a given, ftc even less so, from my understanding) the logistics of commanding an army break down. If you can only communicate at light speed the entire structure of a military organisation breaks down over those distances.
This means that centralized authoity figures for a war are impractical, so you end up with individual generals commanding the forces in a single system. It certainly doesn't mean that you can't have a military.
Rome was quite effective even when its generals had to wait months for messages from Rome. There was nothing about their system that would make an even longer wait less effective.
You idiot, Numsgil. You'd win the war, yes, but it would be an entirely pointless war; a war waged purely for the sake of destroying the other race. I know no examples of that sort of war. The Crusades were a war of territory; the Europeans wanted the Holy Lands. The Cold War was a war of ideaology; each side wanted to force the other to run the world the way they thought it should be run. Never has war been waged for the sole purpose of destroying other human beings.
Even when people have had ethic prejudices strong enough to go to war they have held back, as each side knows instinctively that without a massive advantage that such a war would be the dirtiest imaginable; each side would fight tooth and nail to prevent themselves being annihilated.
That's right, people want territory. Space for their own kind to expand into.
Let's say two interstellar human loose confederacies are at war with one another. One is decentralized, the other has a single urban world and lots of smaller worlds supporting it.
The decentralized people would have an easy time of it. Lay seige on the urban world and within a few years you have entirely crippled your enemy. The remaining rural worlds are easy to take. The urban world is empty, ready to bring in your own people.
People can be very cruel as long as its anonymous cruelty. Seige from space is as anonymous as you can get.
You can see then that urban worlds are a very vulnerable military target. When your done the world is left intact sans the people. Then you can do anything you like with the planet. Rip it up, transport your own colonsits there, whatever you want.
The centralized people are going to have to take not one, but many worlds before their enemies become weakened. Every new planet they find is going to have to be conquered, none will be easy pickings.
The economies of scale, also, simply have a limit before their effectiveness becomes choked by beurocracy. Planet wide specialization is impractical.
Which means that their will be a constant tug for societies to remain as decentralized as possible. Overly centralized societies will collapse as the more efficient, decentralized societies overcome them through war or economic domination.