The phrase "insure domestic tranquility," which is found in the Preamble to the United States Constitution, refers to the need to keep the peace and the assurance that people can conduct their lives and business without fear of social disorder, rebellions and riots. The 18th century framers of the Constitution had good reason to be concerned about disruptions of domestic tranquility because the dangers of social disorder were becoming increasingly evident within the new states. The cost of the Revolutionary War left the states financially impoverished and facing economically fueled outbreaks of violence between farmers, merchants, banks and local authorities.
As social disorder grew more common in the states, it became clear that the limited powers originally given to the central government were inadequate to the task of insuring domestic tranquility. While insolvent farmers' lands were being seized and auctioned off by banks that had loaned them money, courthouses were being burned down in protest.
One of the most alarming disruptions to domestic tranquility took place in the state of Massachusetts in the form of Shay's Rebellion and the resultant bloody confrontation that occurred on January 25, 1787. The inability of the Massachusetts state government to maintain order during the rebellion was perceived as the former New England birthplace of republicanism having fallen prey to anarchy. These events led to the creation of a new U.S. Constitution that gave the federal government increased powers to intervene in states' affairs when it became necessary to insure and/or restore domestic tranquility.
In the modern world, some examples of the "domestic tranquility" referred to in the Preamble to the U.S. Constitution include a central government's ability to protect peaceful demonstrations and assemblies, enforce laws, deal with terrorism and provide citizens with a peaceful means of addressing grievances.