While each state government within the United States holds legal and administrative jurisdiction within its bounds, they are not sovereign in the Westphalian sense in international law which says that each State has sovereignty over its territory and domestic affairs, to the exclusion of all external powers, on the principle of non-interference in another State's domestic affairs, and that each State (no matter how large or small) is equal in international law. Additionally, the member states of the United States do not possess international legal sovereignty, meaning that they are not recognized by other sovereign States such as, for example, France, Germany or the United Kingdom, nor do they possess full interdependence sovereignty (a term popularized by international relations professor Stephen D. Krasner), meaning that they cannot control movement of persons across state borders. The idea of "dual sovereignty" or "separate sovereigns" is derived from the 10th Amendment to the Constitution, which states that "the powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people." Structured in accordance with state law (including state constitutions and state statutes), state governments share the same structural model as the federal system, with three branches of government—executive, legislative, and judicial.
The governments of the 13 states that formed the original Union under the Constitution trace their roots back to the colonial governments of the Thirteen Colonies. Most of the states admitted to the Union after the original 13 have been formed from organized territories established and governed by Congress in accord with its plenary power under Article IV, Section 3, Clause 2 of the Constitution. Six subsequent states were never an organized territory of the federal government, or part of one, before being admitted to the Union. Three were set off from an already existing state: Kentucky (1792, from Virginia), Maine (1820, from Massachusetts), and West Virginia (1863, from Virginia). Two were sovereign states at the time of their admission: Texas (1845, previously the Republic of Texas), and Vermont (1791, previously the De facto but unrecognized Vermont Republic). One was established from unorganized territory: California (1850, from land ceded to the United States by Mexico in 1848 under the terms of the Treaty of Guadalupe Hidalgo).