The principle that each state excludes external actors from its domestic authority structures; named after the Peace of Westphalia (1648), though historians dispute that the treaties established it.
In more detail
Stephen Krasner used the term for one of four meanings of sovereignty, alongside domestic, interdependence and international legal sovereignty. Westphalian sovereignty is breached by intervention (coercive imposition of institutions) and by invitation (voluntary acceptance of external authority, as in human-rights courts or conditional loans). Krasner argued that such breaches are routine. Andreas Osiander and others argue that the 1648 treaties preserved the layered constitution of the Holy Roman Empire, and that the Westphalian model is a nineteenth- and twentieth-century reconstruction.