The debate on Scottish independence includes an argument over whether Scotland can unilaterally decide to leave the union or whether it needs the agreement of the United Kingdom parliament at Westminster to do so.
I'm not expert enough to give an opinion on the legal and constitutional aspects but it seems to me that historically secession only really works where it is agreed on both sides. There is a long list of states which have unilaterally seceeded sparking conflicts, armed or otherwise: the Confederate States of America, Bangladesh, Yugoslavia, Rhodesia.
I'm not saying that countries should obtain the permission of the states they are unwilling to be part of any longer before they secede. Ireland clearly had a right to take up arms in in the 1919 War of Independence that led to the Anglo-Irish Treaty in 1921 and the secession of 26 counties as the Irish Free State. But it would obviously have been better if Britain had agreed - and it clearly wasn't going to without a fight - to respect the will of the majority of the Irish people as expressed by Dáil Éireann's Declaration of Independence and let Ireland secede from the United Kingdom.
The chances of an armed conflict if Scotland holds a referendum and declares itself independent without the go-ahead of Westminster are slim but there would presumably be diplomatic and financial repercussions. The best model for separation (something I still think is unnecessary and unlikely to happen) would be the so-called Velvet Divorce of 1993 in which Czechoslovakia agreed to split into the Czech Republic and Slovakia.