The history of state immunity in international law is extremely storied. It basically had to do with the increase of international trade between nations in the 19th century. As states descended into the marketplace and started to behave like private parties, it was viewed that it was unfair to cloak them with absolute immunity for missing out on a contract or for violating terms of a contractual agreement.
As a result, the first exceptions to immunity were developed. Others now exist. The Supreme Court pointed out that the list is not closed. In fact, our position is developing new and emerging exceptions for precisely what we are talking about here today: crimes against humanity, torture, and crimes of universal jurisdiction.