The world we live in is big, and there are a lot of different people in it. Some of them are smart, and some of them are stupid. Some of them are leftists, some of them are rightists.

There is no law of the universe that precludes smart people from believing in ideologies or causes that you personally dismiss. There are, undoubtedly, militant right-wing militia members who have a very strong grasp of military strategy, insurgency strategy, insurgent tactics, and are planning to use these to fight their enemies.

By their very nature, you will not know about these people. They will not occupy a federal office in a dramatic attempt to coerce the state into doing what they want it to do. They will be completely invisible until they decide to act, and when they do, it might not even be clear that they are acting if they don't want that.

Intelligent people who are happy and healthy and well-fed rarely turn to these sorts of things, but the world is changing rapidly, and there are no guarantees what the future will bring. Further, since the Internet is essentially causing massive global-scale group polarization, it might be much more likely for an intelligent person who might otherwise hold moderate beliefs to be radicalized. It can happen, and because it has billions of chances to, it almost certainly will.