Final answer:
The quote is from the Articles of Confederation, the first constitution of the United States, which outlined the 'firm league of friendship' among the 13 original states.
Step-by-step explanation:
The quote '...a firm league of friendship with each other...' is from the Articles of Confederation which was the first constitution of the United States. These articles were created as a governing agreement among the 13 original states, after their independence from Britain. The states, preferring to retain their sovereignty rather than forming a strong national government, agreed to a 'firm league of friendship' where they committed to mutual defense and freedom.
Learn more about Articles of Confederation