The Boston Massacre, which occurred on March 5, 1770, was a critical event that galvanized colonial resistance against British rule. It involved British soldiers firing into a crowd of colonists, killing five men, which heightened tensions between the Colonies and Britain. The incident was used as a powerful propaganda tool by leaders like Samuel Adams to unite the colonies in their opposition to British policies. It symbolized the growing open hostility and set the stage for the American Revolution.