The event in July 1776 that marked the formal separation of the American colonies from British rule was the adoption of the Declaration of Independence on July 4. Drafted by Thomas Jefferson and other Founding Fathers, this document articulated the colonies' grievances against the British crown and asserted their right to self-governance. This pivotal moment signified the birth of a new nation and established principles of individual rights, liberty, and democracy, shaping the future trajectory of government and society in the United States.