The Judicial power of the United States shall not be construed to extend to any suit in law or equity, commenced or prosecuted against one of the United States by Citizens of another State, or by Citizens or Subjects of any Foreign State. (source)
The eleventh amendment was the first amendment done after the Bill of Rights and the ratification of the Constitution. It was proposed on March 4, 1794 and ratified on February 7, 1795. So it took nearly a year to pass through Congress and get ratified by the states.
This amendment is often debated because of the range of different takes on how to follow this amendment. Does this mean that the state governments can not be sued by their own citizens, or does it just bar out of state citizens and foreign parties? Does it restrict what cases the federal government can view if there is an appeal.
At the very least it protects States from being sued by citizens not their own, and of being named in suits from foreign groups. Debate on how broad or narrow this amendment is continues to this day. It is connected to aspects of the fourteenth amendment, so that tailers what this amendment is read as.