The conflict in Northern Ireland, which has killed thousands, has political and religious roots that are centuries old.
In modern times the conflict is centred on opposing views of the area's status.
Some people in Northern Ireland, especially the mainly Protestant Unionist community, believe it should remain part of the United Kingdom.
Others, particularly the mainly Catholic Nationalist community, believe it should leave the UK and become part of the Republic of Ireland.
Since the 12th Century constant revolts challenged the often brutal British rule of Ireland, climaxing in the 1916 Easter Uprising in Dublin.
It sparked a chain of events leading to civil war and partition of the island.
In the south 26 counties formed a separate state, while six counties in the north stayed within the UK.
Over successive decades the Catholic minority there suffered discrimination over housing and jobs, which fuelled bitter resentment.
In 1969 Catholic civil rights marches and counter-protests by Protestant loyalists (as in "loyal" to the British Crown) spiralled into violent unrest.
British troops were sent in but soon came into conflict with the Provisional IRA (Irish Republican Army).
Loyalist paramilitary groups responded with a campaign of sectarian violence against the Catholic community.
As the situation worsened, Northern Ireland's parliament was suspended and direct rule imposed from London.
Throughout the 1970s, 1980s and early 1990s paramilitary groups waged violent campaigns to pursue their goals.
The IRA carried out deadly bomb and gun attacks in Britain and Northern Ireland that targeted police, soldiers, politicians and civilians.
Loyalist paramilitaries targeted Catholics in "tit-for-tat" killings.
Police and British forces tried to keep order, sometimes amid controversy, such as the alleged co-operation of some undercover units with loyalist groups.
In the early 1990s negotiations took place between political parties and the British and Irish governments.
After several years of talks IRA and loyalist ceasefires held and in 1998 the "Good Friday" agreement was signed.
It set up a power-sharing executive, with ministerial posts distributed by party strength, and elected assembly.
The deal was backed by voters in referendums in Northern Ireland and the Republic, which scrapped its constitutional claim to the north.
Problems remain as devolution has been suspended several times since it began.
Unionists want the IRA to verifiably scrap its weapons and end activity such as so-called punishment beatings.
With the IRA claiming it will do both, Republicans say Unionist demands for pictures of weapons decommissioning are an excuse to avoid power-sharing.
Northern Ireland society has hurdles to overcome too, with views still divided over issues like policing, and a sharply rising crime rate.