The newly independent United States and the Lenape Nation signed a treaty of alliance called the Treaty of Fort Pitt on September 17, 1778. The treaty aimed to unite both parties in a fight against the British who still claimed the 13 American colonies. However, the treaty would not last as its terms were subsequently violated by the US, including the establishment of a 14th state for Native Americans. Learn more about the Treaty of Fort Pitt and the history of the Lenape (Delaware) people.

On September 17, 1787, the United States Constitution was adopted by the 39 delegates to the federal Constitutional Convention in Philadelphia.

Learn more about the Constitution from the National Constitution Center.

On September 17, 1990, Saudi Arabia and the Soviet Union reopened diplomatic ties.