in the U.S preamble what does "insure domestic Tranquility" mean?

Answer 1

to insure (third-person singular simple present insures, present participle insuring, simple past insured, past participle insured)

To provide for mitigation if something goes wrong. Often agree by contract to offer financial compensation in case of an accident, theft or other undesirable event.
I'm not insured against burglary.
Common misspelling of assure.
Common misspelling of ensure. (British English)
To assure, i.e. to promise with emphasis.
He insured me that there would be no further delays.
To ensure. (American English)
Insure that it does not happen again.


domestic (comparative more domestic, superlative most domestic)

Positive
domestic
Comparative
more domestic
Superlative
most domestic


Of or related to the home
1994: George Whitmore, Getting Rid of Robert in Violet Quill - “Dan’s not as domestic as you," I commented rather nastily.
Of or related to activities normally associated with the home, wherever they actually occur
See, for example, domestic violence, domestic hot water
(Of a domesticated animal) kept by someone, for example as a farm animal or a pet.
1890: US Bureau of Animal Industry, Annual report v 6/7, 1889/90 - It shall be the duty of any owner or person in charge of any domestic animal or animals.
Internal to a specific country
1996 Robert O. Keohane, Helen V. Milner, Internationalization and Domestic Politics - The proportion of international economic flows relative to domestic ones.


tranquility

the state of being tranquil
the absence of disturbance; peacefulness
the absence of stress; serenity
the quality of calm such as that experienced in places with mainly natural features and activities, free from disturbance from manmade ones

Answer 2

That was a good answer, but to simplify, it means" To make sure there is peace in our land."