Yes, that may be so, but it is known that Cahokia had a trade in stone tools of Mill Valley Chert that were exported.
The main point of mine being, a civilization has certain hallmarks that make it a civilization. In particular, an agrarian civilization with cities and massive projects (mounds) that require extensive manpower to build. Those cost money and cannot be done for free, so to speak. Even if you use compulsive or slave labor, it has to be paid for. And once you build these massive structures, they become a beacon (and, thus, a target) for everyone who sees them and realizes that, guess what, you've got wealth to spare to build them in the first place.
Further along, it requires a centralized government. And once you've got a government in an agrarian, urban civilization, you've got to have an army to defend that. Every civilization on Earth from the beginning has had one and there is no reason to suspect that Cahokia was different. A hunter-gatherer tribe can pick up and move when threatened by a rival tribe. An agrarian civilization with cities cannot do that. They have absolutely got to defend their cities and land or they die. To do that, they've got to have an effective military force. It's a constant that any ancient civilization that had cities and farmed also had a professional army and a government (and taxes.) The land and cities must be defended and how do you do that? Someone has to make decisions and plan. That means a government. Now how does that government defend their civilization and remain in power? They need an army and just some ragtag militia isn't effective in the long run. So, a professional army is formed, but now it needs to be paid for. Plus, we want to erect these awesome mounds for our Great Sun and temples. How do we pay for that? We've got to have taxes.
As far as evidence of wars, since they left no written records, we can't prove they didn't. For all we know, some field where arrowheads turn up every rainstorm could have been a battle site. Do we know every site where ancient battles took place in Eurasia when the Indo-Europeans tribes were vying with each other for power? No, we don't. But we know they eventually had some people that conquered certain areas and then they probably re-wrote that history in their own oral legends and myths so that by the time it was written down, what they said wasn't what actually happened. But once they established civilizations, they all created professional armies.
My other point is, a lot of the common traits of civilizations tend to be discounted in regards to Native American civilizations. The trend tends to be towards what the Plains tribes did and that tends to color the way other tribes are looked at. If you have a urban civilization, you have to have a government, taxes, and an army to survive be you an ancient Egyptian, a Hittite, a Greek, a Roman, or a Native American. If you're surrounded by hostiles, you can't have something worth taking and expect to keep it without an army, a way to pay for the army, and a government to administer it. Certainly not for a few hundred years. Nothing changes in humanity, only the technology and kill ratio of the weapons. Even today, look around. Certain countries are surrounded by hostile nations and they're all scrambling to find weapons and beef up their armies. This isn't a modern phenomena. It's ancient and is the price paid for having a civilization full of stuff that the "have-nots" desire and figure they can take with enough warriors.
It's like the pyramids in Egypt. Everyone wants to find these fantastic reasons those were built. Aliens from outer space must have done it. How could people have done this thousands of years ago? Quite simply, actually. They had a very rich agricultural system that gave them a bounty of food and, ergo, a surplus population. They had the manpower to do it. They had the money to pay for it. And they had the government to decide, hey, we need these pyramids built over here, so send out officials to tell all the men to show up here and get to work. But why? Well, why did we build the Washington Monument, the Lincoln Memorial, and so on? What purpose do those serve save for self-aggrandizement and monuments to the "Hey, look at us! We're a great civilization, see these things we built to prove it?" Governments do that kind of thing. They always have. It demonstrates power. So, when you have a culture called the Moundbuilders, you can see they were doing this to testify to the greatness of their civilization. To be honest, most of those mounds weren't mounds at all, but pyramids. So, there we have it.