Union soldiers 1863.
How easy might it be for Texas, or Scotland, or Western Ukraine to become independent?
In 1861, the USA fought a civil war to prevent a break up of the union.
Since the 19th century, the Kurds have been trying to break away from Turkey. 35,000 people have been killed in the recent Turkish-Kurdish conflict.
In 1905, Norway voted in a referendum to become independent from Sweden.
Sweden briefly considered war.
Then the Swedes let Norway break away peacefully.
1916 Easter Rising in Dublin
Britain put up a fight to prevent Irish independence, but eventually gave independence to the South in 1921.
Britain has used violence to prevent the North from becoming independent.
In 1956, the Russians used violence to put down the Hungarian revolt.
In the 1968, the Russians used violence to put down the revolt in Czechoslovakia.
In 1991, the break-up of the Soviet Union was peaceful.
However, the Russians have fought a war to prevent Chechnya from becoming independent.
In 1993, Czechoslovakia was peacefully split into two countries, one for the Czechs and one for the Slovaks.
Alex Salmond's plans.
A Chinese diplomat told journalist Gideon Rachman that "England would never accept Scottish independence. It would invade."
Scotland can be a model for how to handle separatism - Gideon Rachman - Financial Times - 17 February 2014.
Rachman writes: "The UK government’s willingness to allow the centuries-old union to be dissolved peacefully is a boost to the country's reputation."
However, it should be noted that the London government, and its allies, are doing everything they can to prevent Scotland becoming independent.
Scotland has strategically important waters, huge energy reserves, and important military bases such as the nuclear submarine base at Faslane.
NATO, London, the EU and the USA would be enormously keen to see an independent Scotland remaining a close ally.
The most likely outcome of the 2014 referendum in Scotland is a 'NO' vote, followed by London giving more powers to the existing Scottish parliament.