Why might the founding fathers have included the phrase “insure domestic tranquillity”

For peace among individuals

To protect the citizens from domestic unrest, is the right answer.

Explanation:

The Framers of the Constitution of the United States had enough reason to attempt to “insure” domestic tranquility. Truly “domestic Tranquility” implies peace and quiet at the domestic level in the United States, in contrast to other countries. By using the phrase Tranquility, the Framers intended the nonexistence of disorders, revolutions, and comparable manifestations of social upheaval. They were hugely concerned with domestic tranquility because social chaos had become a more timid, vulnerable, and general state of activities in the new nations. Thus to protect the people of the state from domestic unrest, the framers used the phrase "insure domestic tranquility”.