Well, because it’s difficult to prevent war. It’s sort of like: “If people still get murdered, why do we bother with a police force? They only get there after the person is dead!”
Hopefully, people will be aware of the consequences of their actions and that knowledge will act as a deterrent, but, ultimately, all of us have free will and the possibilty to do stupid things, and at the end of the day, you can’t regulate stupidity. You can curb it, a lot, but you can’t remove it, and history shows that the more you try to regulate it, the worse the explosion gets once the system falls.
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