Never again
After World War II, people introduced the phrase "never again". Never again should the whole world fight a war against each other. Never again should Germany, Japan, and Italy attempt to get rid of people from other countries. Or as Elie Wiesel wrote: "Never again' becomes more than a slogan: It's a prayer, a promise, a vow ... never again the glorification of base, ugly, dark violence."
As a thought experiment, how can we ensure that this never happens again? "territorial integrity" seems to me like a great start. Allowing nations (read: groups of people) defend their border to me seems to go a long way in protecting people from each other. Sovereignty also plays an important role. It ensures that people are allowed to rule themselves. What is the point of a fixed border if someone else makes decisions for you?