The soldiers were the first servicemen to be killed in NI in 12 years
A policeman has been shot dead in County Armagh - the first PSNI officer to be murdered by paramilitaries since the force was formed in 2001.
The killing follows an attack by the Real IRA outside an army base on Saturday, in which two soldiers died and four people injured.
The attacks, are the latest incident in a conflict with political and religious roots that goes back centuries.
What is Northern Ireland's history?
Northern Ireland was formed in 1921 after its population disagreed about whether to become part of Britain or part of Ireland.
Broadly the places that had a majority in favour of the union with Britain would form the body of Northern Ireland, which was then included in the UK.
Why is there conflict?
The main issue still centres on whether it should be part of the UK or Ireland.
Some people, 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.
Relationships between the two sides reached breaking point in the late 1960s when The Troubles started; violent uprisings, riots in the streets and bombing campaigns which lasted more than 30 years.
To date more than 3,600 people have died in the conflict - most of them civilians.
Why are the Army in Northern Ireland?
The Army were deployed to keep the peace after it was felt that the Royal Ulster Constabulary (RUC) had lost control of the situation.
But the Army were soon seen as an occupying force by many nationalists. This made them targets for paramilitary groups.
At the height of the Troubles there were 28,000 soldiers in Northern Ireland.
The operational mission of the UK armed forces in Northern Ireland ended in July 2007.
Some 5,000 troops remain on garrison duty - living and training as they would on any base.
What have the politicians been doing about it?
The peace process has been a work in progress for the past 20 years.
Negotiations took place between the British and Irish governments and politicians called for a ceasefire.
After ceasefires by the Irish Republican Army (IRA) and loyalist groups, negotiations between political parties led to the Good Friday Agreement in 1998.
In 2007 devolved government was restored when the two opposing parties Sinn Fein, a republican party, and the Democratic Unionist Party (DUP), reached a deal.
These parties now dominate politics in Northern Ireland and work alongside each other.
First Minister Peter Robinson is a member of the DUP and Martin McGuinness, a former IRA commander, is Deputy First Minister.
It has been reported that the Real IRA (RIRA) has been "especially active"
Who is still against the peace process?
Security experts say dissident republicans now represent the biggest danger.
A watchdog which monitors paramilitary groups has said several are still active.
These are Continuity Irish Republican Army (CIRA) and the Real Irish Republican Army (RIRA), the Irish National Liberation Army (INLA) and ”glaigh na h…ireann.
It is reported that CIRA and RIRA have been "especially active" and are thought to still be preparing and planning more attacks.