When the founders wrote "to form a more perfect Union", they meant that a Bill of Rights would be created to protect citizens liberties.
The Constitution of the United States created by the Founding Fathers is a bill of rights, with the influence of the Magna Carta, with which they sought to establish the rights of the American people and their States, and protect citizens liberties, in order to guarantee the peaceful and harmonious coexistence of the society. From there it is explained that when the preamble speaks of "form a more perfect Union", it does it in the sense of guaranteeing peace among the members of the country.
