The United States is separated into 50 semi-autonomous states, each of which has its own constitution and government. The Federal Government has the authority to establish a state, but there are no provisions in the Constitution to require the state to remain in the Union; however, the precedent set by the Civil War indicates that secession by a state will be greeted by invasion.
The Constitution of The United States delegates to the states, by way of the Tenth Amendment, any power not vested in the federal government, thus creating sovereign state governments. Nevertheless, the federal government has been able to influence state laws through financial incentives, and in some cases, by creating federal laws that the Constitution does not allow it to create. Through the power of Judicial Review, the Supreme Court has removed laws that infringe on the sovereignty of the individual states.