A sovereign nation is one that is independent and has supreme authority or power over itself. The United States is a sovereign nation, declaring itself to be so when, in the Declaration of Independence, the forefathers wrote of Americans as "one people."
A sovereign nation is ruled by its own people. One definition of the word "sovereign" is supreme ruler. In a sovereign nation, the nation is its own supreme ruler.
In the words of the Declaration of Independence, America has the right to be free and independent. Its independence from foreign rule and practice of self-government establish it as a sovereign nation.