How is the Peace of Westphalia important today?

The 1648 Peace Treaty of Westphalia is important today in this respect: it articulated in the first formal way (in the context of a diplomatic congress of contending nations) the principle that sovereign nations are not rightly interfered with in respect to domestic affairs. While many years would pass before this articulation would transform into a fully recognized (and authoritative) rule in the world-community, it was a remarkable first step towards what is common in contemporary international relations.