The significance of the First and Second Continental Congresses in American history lies in their role in uniting the colonies against British rule. The First Continental Congress convened in 1774 to address colonial grievances and assert rights, leading to the formation of a unified resistance. The Second Continental Congress, occurring in 1775, took more decisive actions, including appointing military leaders like George Washington and drafting the Declaration of Independence. Both congresses cultivated a spirit of collaboration and political unity among the colonies, laying the groundwork for the eventual fight for independence and the formation of the United States.