The act of starting a war should be a violation of international criminal law. This is because of the fact that war is more and more terrible as technology advances in the world. War is now something that is continually destroying the world and our people so we should make it a crime to help end war.
I believe violence and envy is in our nature as human being and can not be avoided. Even if we do everything that we can to make war illegal, people will find a way or a reason to kill one another. Can you get rid of famine, inequality, hate, greed, pride and/or tribal skepticism? All those, and more, lead to war.
There is a doctrine of just war theory, usually humanitarian intervention comes up, wherein an aggressor not only has a right - but indeed an universial ethical duty - to delcare war and stop the atrocities. Easy examples include Rwanda, Darfur, South Sudan, or the humanitarian crises in North Korea.
For centuries there have been wars and countries,territories, and regions have been starting wars against another. War is a neccessary sometimes if another country, territory, or region invades another or threatens another and its people. The other must have a way to relatilate and protect itself and its people from the enemy.