What is the power of a nation-state to determine its own goverment

Sovereignty

A nation-state's sovereignty applies not only to determining its own form of government but also to being free from interference by outside powers.  The principle of sovereignty was established in Western history by the Treaty of Westphalia in 1648, at the end of the 30 Years War.  The principle recognized the right of rulers to govern their own territories and not be controlled by other powers outside their territories.