Domestic tranquility literally means "peace at home." This phrase appears in the preamble to the U.S. Constitution as a quality to be promoted and a reason for the very establishment of the Constitution.
In the early days, after becoming a nation, violence was not uncommon in the United States. Economic unrest and the absence of a strong centralized government were largely to blame for the rioting and rebellions that occurred all too often. Particularly at odds with each other were the overburdened farmers and wealthy merchants. The writers of the Constitution wanted to bring about lasting domestic tranquility and considered it important enough to mention it early in the document. In modern times, the U.S. government continues to strive for domestic tranquility, as it squelches terrorism within its borders and manages peaceful demonstrations.