Every school child learns about the Preamble to the U.S. Constitution at one time or another. They learn how we the people wanted to form a more perfect union, establish justice and insure domestic tranquility, among other worthy endeavors. A "more perfect union" can be contrasted with a less perfect union and thus admired without much thought, while establishing justice seems a good cause to most people. But what exactly does domestic tranquility mean?
TL;DR (Too Long; Didn't Read)
When the framers put the words "domestic tranquility" into the Preamble of the Constitution, they wanted to make sure that the federal government had the authority and power to prevent or stop quarrels and fighting among the states. The term is currently used to mean a country at peace from social strife and rebellion.
The framers of the Constitution lived in a very different age from our own. The United States was just forming, and the new nation's safety and stability would require a certain level of peace and good will as opposed to open fighting or rebellion within and among the individual states. That wasn't always the case. Just before the Constitutional Convention of 1787, a year-long series of violent protests had taken place in Massachusetts and other states. Farmers protested against state and local enforcement of tax collections and judgments for debt, and the protest was given the name Shay's Rebellion.
Shay's Rebellion was named after its leader, Daniel Shays of Massachusetts, a farmer and former captain in the Continental army. The rural population, mostly poor farmers, believed that the merchant class was imposing unfair economic terms on them by assessing taxes. Led by Shays, some 4,000 rebels armed themselves and tried to take over the federal arsenal at Springfield, attacking merchants, lawyers and supporters of the state government. The uprising was crushed in the winter of 1787.
The politicians drawing up the U.S. Constitution were truly alarmed by Shays’ Rebellion. They wanted to be sure that the federal government had the authority and power to stop these sorts of uprisings by force or persuasion. They also wanted the government to be able to smooth out territorial disputes among states and keep them from warring with one another.
The term domestic tranquility is generally used in the modern world to mean the government's inherent ability to protect and encourage peaceful demonstrations and assemblies. Rather than being interpreted as allowing the government to quash all protests as it enforces the laws of the land, the U.S. government's job is to make certain that all citizens have a peaceful means of addressing grievances.
Some believe that domestic tranquility also implies protecting the country from its foreign enemies. This includes maintaining good relations with our allies and being watchful of those countries that wish to do harm to the United States. However, some laws passed in the name of ensuring domestic tranquility are much criticized today, including the internment of Japanese-American citizens during World War II and the Patriot Act, which many claim breaches basic constitutional rights.