'The Troubles' generally refers to the roughly 30-year period of violence and political dispute in Ireland that spanned from the late 1960s to the late 1990s.
It is perhaps most well-known for 'Bloody Sunday' in January 1972, when 13 people were killed by soldiers of 1st Battalion of the Parachute Regiment during a civil rights march.
One other man died several months later from injuries believed to have been sustained on Bloody Sunday.
The roots of the Troubles can largely be traced back to the 17th century and involve overlapping conflicts over religion and nationalism, and, later, the history of grievances these contests caused.
In the early 1600s, the Catholic and Protestant wings of Christianity were at war, and Protestant Scottish and English settlers were encouraged to set up home in the north of Ireland as a buffer against Catholicism there.
England’s enemies, the French and Spanish, were also Catholic, and the fear was that they might link up with and encourage Catholic uprisings in Ireland.
Naturally, this in and of itself led to war.
The first major conflict was the Irish Confederate Wars from 1641 to 1652, and the second was the Williamite War of 1689 to 1691.
These resulted in Protestant victories, but they also stored up problems for centuries to come.
The Orange Order was established in 1795 to bind together the various Protestant factions and it gave them improved political clout.
The 1801 Act of Union abolished the Irish Parliament and brought Ireland and Great Britain together, seeing the United Kingdom spread across the British Isles for the first time.
Catholics, who were 75% of the population of Ireland, were not politically free at first, and although they achieved emancipation in 1829, efforts by many to reverse the Act of Union went nowhere - at least at first.
By the late 19th century, a movement for Home Rule had gathered steam and advocated for an Irish Parliament to rule Ireland under the official authority of the parliament in Westminster.
The issue split Victorian Britain, with Tories opposed to it, and the differing strands of the Liberal Party in disagreement.
Eventually, the Liberal Unionist Party would form, breaking away from its parent body to side with the Tories in opposition.
There was even stronger opposition in Ireland itself.
In 1914, as Herbert Asquith’s ruling Liberal government considered going forward with Home Rule, arms smuggled from Germany were supplied to the militant Ulster Volunteer Force (UVF) and things began to heat up.
But then the First World War broke out. This made it even harder for both sides to be reconciled.
In 1916, the Easter Rising against British rule was led by those who wanted not just Home Rule, but full Irish independence.
Any disagreement the Catholic majority may have had with these revolt leaders about full independence was largely cancelled in the aftermath.
The decision by the British to execute the ringleaders led to an increase in public sympathy for their cause.
By 1918, forces were also pulling more strongly in the opposite direction.
This is because it became politically unthinkable to ‘betray’ the sacrifices made by those in the 36 Ulster Division during the war through British support of Home Rule or Irish independence.
In the years that followed the First World War, Ireland became a flashpoint for violence as its political future was fought over.
By 1922, after the Irish War for Independence, Ireland had been divided in two, with the 26 counties in the south forming the Irish Free State (later the Republic of Ireland), and the six counties in the north remaining a part of the United Kingdom.
But the question of national boundaries had still not been settled, with the year-long Irish Civil War ensuing between the republican and nationalist factions of the Irish Catholic forces that had fought during the war for independence.
The republicans rejected the new boundaries while the nationalists, who were supported by the British government, supported the arrangement.
The republicans lost the war, but the resurgent IRA of later years came out of the desire by this side for a fully united Ireland that was completely independent from Great Britain.
Despite the IRA’s commitment to this cause, relative peace was maintained until the late 1960s.
This was when long stored-up political grievances came to the surface.
In Northern Ireland, the two-thirds Protestant majority saw the Catholic minority as a threat and had consolidated political power to the point that Catholics in the north no longer had an effective voice.
David McKittrick and David McVea describe how this process came about in 'Making Sense of the Troubles: A History of the Northern Ireland Conflict': "From the start, the Unionist party’s leaders believed that the new state [of Northern Ireland] could only survive if the levers of power were firmly in reliable Protestant hands.
"The first instincts of Unionists, having been put in charge by Westminster, were to ensure that their power should be both undiluted and permanent.
"Thus one of the new government’s earliest acts was to set about changing the voting system and local council boundaries inherited by the new Unionist government."
This resulted in Catholics being marginalised and, like the US and South African civil rights movements at that time, a movement for Catholic rights in Northern Ireland took off in 1968.
It was from this effort that violence escalated.
In 1969, with police in Northern Ireland under pressure from protests, British troops were deployed to assist them.
In 1971, Home Secretary Reginald Maudling dubbed the struggles of this period "an acceptable level of violence". This was later interpreted by Unionist politicians as a remit for a certain level of paramilitary activity to contain the problems.
But in 1972, the attempt to contain protests went badly wrong.
The infamous Bloody Sunday, or Bogside Massacre, occurred in Londonderry, the second largest city in Northern Ireland.
British soldiers in 1 PARA were dispatched to deal with protestors.
A policy of internment without trial had come into effect in 1971 in response to increasing levels of violence across Northern Ireland, but this, in turn, fed further protests.
When protestors and paratroopers clashed in Londonderry, 28 people were shot and 13 died, with one more person dying months later.
While Bloody Sunday is one of the most prominent events of the Troubles, the death toll from it was only the tip of a much larger iceberg.
According to the Conflict Archive on the Internet, a total of 51,000 people became casualties before the Good Friday Agreement in 1998.