The dispute over whether Iraq is in the grip of a civil war has surfaced again, with the UK Foreign Secretary Jack Straw and his Saudi counterpart airing directly contrary views during a conference in Riyadh.
Sir Lawrence Freedman, Professor of War Studies at King's College London examines the historical precedents and why the argument matters so much.
When does sectarian violence become civil war? That question is being posed almost daily in the face of the steady deterioration of the security situation in Iraq since the election at the end of last year.
Will Iraq's police force be left to deal with a civil war?
The question is not just a matter of definition.
If the problem is sectarian violence then there is still hope that if a broadly-based government can be agreed, then it might get a grip on the situation.
Coalition troops working with the local Iraqi forces could help bring the country back to a semblance of order, helping to keep the peace between the rival communities.
But if it is really a civil war then there is no possibility that such a government will be formed. The police and armed forces will be seen as the partial instruments of the Shia majority and will soon become just one militia among many.
The coalition forces might as well give up and go home.
Iraq's future will be decided by battle rather than through political negotiation.
This is why the description has been so fiercely disputed by the American, British and Iraqi governments.
American civil war
Strictly speaking a civil war takes place within the same political community and represents a struggle for power between competing factions that is decided through violence.
The American civil war fits the category as it was about one part of the country's attempt to break away from the rest and it involved a clash between organised armies.
After the Bolshevik revolution there was a civil war in Russia as the 'white' forces loyal to the old order fought the 'red' forces of the new.
Pronounced differences in religion or ethnicity may produce a civil war if one community decides it is intolerable to live in a state dominated by another.
The Iraqi government can't provide basic security to its citizens
The militants will use sectarian attacks in order to polarise the country and undermine any claims by the state to be representing all sections of the population.
The Irish Civil War refers to the internal struggle that took place in the 1920s between factions of the Republican Movement over whether or not to accept partition.
This would have given them their own state in the South but left the six counties of the North controlled by Protestants determined to maintain the union with Great Britain.
When the Provisional IRA mounted a violent campaign to separate the North from Great Britain and attach it to the South they might have wished this to appear as a civil war.
Yet in practice it was seen as a vicious reflection of the Catholic-Protestant sectarian divide.
There were paramilitaries operating on behalf of both communities, but they were never in a position to defeat the forces of the state - the police backed by the armed forces - though they were able to make the search for a political settlement appear more urgent.
One possibility with chronic inter-communal violence is not that it leads to civil war, in which it is possible for one side to defeat the other in battle, but instead to a complete breakdown of social order so that there is no effective government at all.
The Lebanese civil war, which began in the mid-1970s and did not really end until 1990, was of that nature.
Within each confessional group could be found a number of militias, who fought each other, and any foreign troops, and could never gain control of the whole country.
Beirut was left in ruins by the long conflict
Countries in which there is no effective central government but local warlords using loyal militias to protect their local interests are not uncommon in the Third World, for example in Somalia.
This is the sort of scenario that is more likely in Iraq than a classical civil war, because the Shia majority should always be able to crush any opposition forces drawn from the minority Sunnis.
A far greater risk is that the levels of routine violence reach such a point that the state, even if still notionally backed by coalition forces, is unable to provide a basic level of security to its people and the country implodes.