Naive Question

The Goldstone Report left me with a naive question. International law defines this, international law defines that, why doesn't the international law define that war is against the law? Instead of defining what weapons and conduct are allowed during war, wouldn't it be simpler to just not allow war at all?

Former military industry workers and soldiers will use former defense budgets to create new and exciting things for humanity. Disputes between countries will be solved by international court and police. Disobeying countries will be sanctioned up to cutting all ties with them.

Don't raise your eyebrows, told you it's naive.