That river rich in history is the Euphrates River. The Tigris River and Euphrates River gave the ancient name "Mesopotamia" ("Land between the Rivers") to a region that was an ancient cradle of civilization. The lengthy Euphrates River originates in eastern Turkey, flows through Syria and Iraq, and then joins the Tigris River in the "Shatt al-Arab" ("River of the Arabs") which flows out into the Persian Gulf.
Answer:
It put the territory under one government.
It ensured religious freedom.
It laid out a way for territories to become states.
It made slavery illegal.
I could be wrong but I believe it's the Cape Floristic Kingdom