Factbox: Northern Ireland suffers worst violence in years

(Reuters) - Riots broke out in Northern Ireland last month after a vote by mostly pro-Irish councilors to end the century-old tradition of flying the British flag daily from Belfast City Hall.

Here is a look the background to Northern Ireland's continuing sectarian divide:

* At least 3,600 people were killed over three decades as Catholic nationalists seeking union with Ireland fought British security forces and mainly Protestant loyalists determined to remain part of the United Kingdom.

* In the late 1960s, tensions between Catholic Republicans and members of the pro-British Unionist majority spilled over into riots. British troops were deployed on the streets. The Irish Republican Army, originally the name given to a militia that fought for Irish independence in the early part of the century, re-emerged. From 1972 the Provisional IRA took over the armed struggle against British rule.

* In a joint Anglo-Irish declaration in 1993, Britain said it would not block an end to British rule in Northern Ireland if a majority wanted it, and offered Sinn Fein a seat at peace talks if the IRA renounced violence. Talks followed. The Good Friday Agreement was signed in April 1998 and created a power-sharing assembly and government for Northern Ireland.

* Disagreements over disarmament and over the establishment of a new police service for the province brought suspensions of devolved rule, but it was definitively restored in 2007 when Protestant Unionist leader Ian Paisley and Sinn Fein's Martin McGuinness were sworn in as first minister and deputy first minister.

* Major violence has ended in the province but much of the city of Belfast remains divided along sectarian lines. Militant nationalists have been responsible for the killings of three police officers and two soldiers since 2009, but have not reacted against the flag protests, reducing a threat to 15 years of peace in Northern Ireland.

(Reporting by David Cutler, London Editorial Reference Unit)