Every undergraduate learns that the modern sovereign state was born at the Peace of Westphalia in 1648 — borders, non-interference, the works. It is a tidy story and largely retrofitted. The treaties say little about sovereignty as we now mean it; the doctrine was read back into them centuries later by people who needed a founding myth.
The more honest history is messier and more useful. Order between polities was never established once and maintained. It was — and is — renegotiated continuously, treaty by treaty, crisis by crisis, each settlement provisional and contingent on the next balance of power. There is no moment after which sovereignty simply exists.
This matters for anyone who takes the sovereign ideal seriously. Imagine sovereignty as a status conferred at a founding and you will defend the wrong thing. Understand it as an ongoing negotiation each generation must actually conduct, and you will tend the right one. Westphalia was never the point. The negotiating was.