The event in 1789 that led to the establishment of the National Assembly in France was the Tennis Court Oath. This pivotal moment occurred on June 20, 1789, when members of the Third Estate, representing the common people, found themselves locked out of a meeting of the Estates-General. Seeking to assert their rights, they convened in a nearby indoor tennis court and vowed not to disband until a new constitution was established for France. This oath symbolized the Third Estate's resolve to challenge the existing power structures of the monarchy and the privileged estates. It marked a crucial step toward the French Revolution and the eventual establishment of a constitutional monarchy.