(CNN)Here's a look at Northern Ireland. For many years, Northern Ireland has been split over the question of whether it should remain part of Great Britain or become part of Ireland.
Nationalists, who are mostly Catholic, would prefer to belong to a single, united Ireland. Most of its Protestants are determined to remain a part of the UK; they are called Unionists.
Northern Ireland's history has been marked by sectarian violence, although in recent years, its militias and parties have been working toward compromise, and the two sides now make up a power-sharing government.
Between 1969 and 1997, sectarian violence left almost 3,600 people dead. The conflict is often called "The Troubles."
About Northern Ireland:
Northern Ireland is one of the Home Nations of the United Kingdom, along with England, Scotland, and Wales.
Belfast is the capital of Northern Ireland.
As of 2011, Catholics make up 45% of the population. Protestants make up about 48%.The other 7% are members of other religions or non-religious.
Political Groups and Militias:
The Democratic Unionist Party (DUP)
The party with the most seats in Northern Ireland parliament and is the main party in a power-sharing government with Sinn Fein.
Formed in 1971 by Ian Paisley, a protestant preacher, the DUP is now led by Peter Robinson, the first minister of Northern Ireland.
Historically, it has attracted support from working-class Protestants and now enjoys support from most of the protestant voting population.
The party began as an opponent to power-sharing between unionists and nationalists, and, under Paisley, it opposed the Good Friday Agreement of 1998.
Ulster Unionist Party (UUP)
Now led by Mike Nesbitt.
The UUP was the dominant unionist party for decades until it was overtaken by the DUP.
Formerly the Official Unionist Party, it formed the government of Northern Ireland from 1921 until 1972, when direct rule from London was imposed.
The party had close links with the Orange Order, a Protestant fraternal organization, in the past.
Social Democratic and Labour Party (SDLP)
Led by Alasdair McDonnell.
A former party leader, John Hume, played a pivotal role in the peace process when he agreed to hold talks with Sinn Fein leader Gerry Adams and with the British Government.
The party attracts middle-class Catholic support as well as some working-class support, and aims to achieve the reunification of Ireland through democratic means.
Is a long-standing critic of all paramilitary groups and the British military presence in Northern Ireland.
Northern Ireland's leading and oldest republican party.
Led by Gerry Adams.
Sinn Fein advocates a united Ireland free from British rule or a British presence.
The party's present form dates back to 1970 when Provisional Sinn Fein split from Official Sinn Fein, which became the Workers' Party.
The political ally of the Provisional IRA, Sinn Fein is a supporter of the Good Friday Agreement.
In 1986, a breakaway group, calling itself Provisional Sinn Fein, was formed which opposed IRA ceasefires and the peace process.
Irish Republican Army
The IRA is the chief republican paramilitary group.
It was founded in the early 1900s to fight for an independent Ireland.
1969 - It splits into the Official IRA and the Provisional IRA (the former had adopted a Marxist political ideology and rejected violence while the latter favored being an armed force).
1972 - Following the Official IRA ceasefire of 1972, the Provisionals become known as the IRA.
September 26, 2005 - The IRA scraps its weapons after more than three decades of armed struggle against British rule.
Real IRA or New IRA
1997 - Is formed when a dozen members of the Provisional IRA break away in protest at Sinn Fein's entry into dialogue with the British and Irish governments.
Aug. 15, 1998 - Omagh bombing kills 29 people. The Real IRA claims responsibility. The group later says it regrets the civilian deaths and calls a ceasefire less than a month later.
Following the split from the IRA, it worked with the Continuity Irish Republican Army (CIRA) and the Irish National Liberation Army (INLA).
The marching season lasts from Easter Monday to the end of September. Most marches are staged by the Orange Order.
The marches commemorate important days in Northern Ireland's history.
The Twelfth of July is an important day for marches. It celebrates Protestant William of Orange's defeat of the Catholic King James II at the Battle of the Boyne.
The parades have been contentious throughout their history.
The Public Processions Act of 1998 enacts the Parade Commission to help monitor, mediate, restrict and review the parades.
Good Friday Agreements/Northern Ireland Assembly:
April 10, 1998 - Agreements signed to create a 108-member Assembly in which both Catholic and Protestant political representatives sit together in government. It is the second time such power-sharing has occurred since 1920. The first was the short-lived Sunningdale Agreement of 1973-74.
June 25, 1998 - 108 members are elected to the new Assembly.
October 14, 2002 - The assembly is suspended for a fourth time, by the British government, amid allegations of espionage by the Irish Republican Army (IRA).
November 26, 2003 - Elections are held for the Assembly. The Democratic Unionist Party (DUP), lead by Ian Paisley, win the most seats.
February 3, 2004 - The parties that won seats in assembly elections gather at Stormont, the seat of government in east Belfast, for the opening session of a review of the agreements that created the power-sharing organization.
September 2004 - More talks between the parties, but the talks end without resolution.
May 8, 2007 - The new Northern Ireland Assembly is inaugurated. DUP founder Ian Paisley is named first minister and Sinn Fein's Martin McGuinness is named deputy first minister.
1920 - The Government of Ireland Act splits the country into two separate political units, a Catholic south and a Protestant north.
1949 - The south cuts all ties with Britain, becoming the independent Republic of Ireland. The six counties of Northern Ireland remain a part of the United Kingdom.
December 1969 - The Irish Republican Army (IRA) splits into two factions: A breakaway group becomes known as the Provisional IRA and the remaining group is renamed the Official IRA.
January 30, 1972 - "Bloody Sunday" - An anti-internment march, organized by the Northern Ireland Civil Rights Assn (NICRA) in Derry turns to rioting. Fourteen people are killed.
March 1972 - British Prime Minister Ted Heath suspends the Northern Ireland Parliament, thus ending Home Rule and imposing Direct Rule from London.
July 21, 1972 - "Bloody Friday" - The IRA explodes 22 bombs, killing 9 people.
March 1973 - The government proposes a 78-member assembly comprising unionist and nationalist representation to take over the affairs of state in Northern Ireland.
May 1974 - The Sunningdale Agreement is approved by the Northern Ireland Assembly, creating a Council of Ireland. A general strike is called by the loyalist Ulster Workers' Council. The strike lasts 14 days and brings Northern Ireland to a standstill, ultimately forcing the collapse of the council.
1970s and 1980s - Bomb attacks take place in various places in Northern Ireland, and many people are killed. The IRA claims responsibility.
August 1979 - Eighteen British soldiers are killed in an IRA attack at Warrenpoint, County Down, in what is the British Army's greatest loss of life in a single attack in Northern Ireland. The same day, Earl Louis Mountbatten, the Queen's cousin, is killed by a booby-trap IRA bomb on his boat near County Sligo in the Republic of Ireland. Three other people are also killed in the explosion.
May 1981 - Bobby Sands, leader of the IRA in Maze prison, dies of a hunger strike. His death sparks riots across Northern Ireland. 100,000 people attend his funeral.
November 1982 - First sitting of the new Northern Ireland Assembly at Stormont, Belfast. The Social Democratic and Labour Party (SDLP) and Sinn Fein (SF) refuse to take up their seats.
November 1985 - Prime Minister Margaret Thatcher and Garret Fitzgerald, the Irish Prime Minister, sign the Anglo-Irish Agreement, that for the first time gives the Irish Government a consultative role in matters related to security, legal affairs, politics, and cross-border co-operation.
March 1986 - Unionists strikes disrupt Northern Ireland.
June 1986 - The Northern Ireland Assembly is dissolved.
January to August 1988 - A series of ground-breaking talks are held between John Hume, the leader of the Social Democratic and Labour Party, and Gerry Adams, President of Sinn Fein.
August 1994 - The IRA announces a ceasefire.
May 1995 - First official meeting in 23 years between Sinn Fein and the British Government.
October 1996 - Two IRA bombs at British Army Headquarters mark first attack against the security forces in Northern Ireland by the IRA since its ceasefire of August 1994.
April 10, 1998 - The Good Friday Agreement is signed, creating a 108-member Assembly and 14-member executive body in which both Catholic and Protestant political representatives sit together in government.
August 15, 1998 - A bomb kills 29 people in Omagh, the single worst incident in the history of the conflict. The Real IRA claims responsibility.
December 1999 - Direct Rule ends as powers are devolved to the Northern Ireland Assembly.
October 2002 - The British government suspends the Northern Ireland Assembly over alleged IRA spying.
February 2003 - The Ulster Defence Association, Northern Ireland's largest and most active Protestant guerrilla organization, announces a 12-month ceasefire.
November 27, 2003 - The Democratic Unionist Party (DUP) and Sinn Fein are the winners of the legislative elections, gaining more seats than other parties. As a result, the British and Irish governments must negotiate primarily with Ian Paisley and Sinn Fein's Gerry Adams.
September 2004 - A new round of talks to try to restore the assembly is held at Leeds Castle in southern England. Although the talks break up without agreement, party leaders and the prime ministers agree to keep talking behind the scenes.
November 2004 - The British and Irish governments put new proposals aimed at securing a breakthrough to the DUP and Sinn Fein. Ian Paisley and Gerry Adams agree to take the plans to their members.
January 30, 2005 - Robert McCartney is killed in a bar brawl with IRA members in Belfast, Northern Ireland. No witnesses come forward, although there were over 50 people in the bar that night. His sisters campaign for an investigation into his murder. Later, Sinn Fein suspends seven members, and the IRA suspends three allegedly involved.
September 26, 2005 - The IRA announces it has decommissioned its weapons.
May 16, 2006 - The Northern Ireland Assembly meets for the first time in three and a half years.
October 11-13, 2006 - Gerry Adams and Ian Paisley meet with Tony Blair and Bertie Ahern in St. Andrews, Scotland to discuss the Assembly.
March 7, 2007 - An election for a new provincial assembly in Northern Ireland takes place. The election could lead to re-establishment of a power-sharing government that collapsed in 2002. After the election, the political parties will have three weeks to reach agreement on formation of a government, or the new assembly will be dissolved.
March 26, 2007 - The first face-to-face talks between Ian Paisley (DUP) and Gerry Adams (Sinn Fein) take place.
May 8, 2007 - Inauguration of the new Northern Ireland Assembly, where Sinn Fein and the Democratic Unionist Party will share power.
August 1, 2007 - The British Army ends its mission in Northern Ireland. The mission, known as Operation Banner, began in 1969 and was designed to support local police. At its highest point, approximately 27,000 troops were stationed in Northern Ireland.
June 2008 - Peter Robinson succeeds Ian Paisley as First Minister of Northern Ireland.
March 2010 - The Northern Ireland legislature ratifies an agreement to "devolve," or bring under local control, the province's police forces.
September 6, 2011 - The trial of 14 alleged members of the pro-British or loyalist paramilitary group known as the Ulster Volunteer Force, who are accused of a total of 97 offenses, begins. Nine defendants are charged with murder in the October 2000 slaying of rival paramilitary leader Tommy English.
January 20, 2012 - Brian Shivers is convicted of the 2009 murders of British soldiers Mark Quinsey and Patrick Azimkar and sentenced to life in prison.
June 27, 2012 - Martin McGuinness, a member of Sinn Fein and Deputy First Minister of Northern Ireland, meets privately with Queen Elizabeth, and later in the day shakes hands publicly.
May 3, 2013 - Shivers is acquitted in a retrial. The DNA evidence found at the scene was not deemed strong enough to connect him to the murders.
June 17-18, 2013 - Lough Erne Resort, in Enniskillen, Northern Ireland, hosts the 39th annual G8 Summit. This is the first time the event is held in Northern Ireland.
December 23, 2014 - After extensive talks, an agreement is reached between Northern Ireland's political parties, the UK, and Ireland, which helps avoid the collapse of the power-sharing government.
May 5, 2015 - Gerard "Jock" Davison, a former senior IRA figure, is shot dead in Belfast.
May 19, 2015 - Sinn Fein head Gerry Adams meets with Prince Charles in Ireland, the first public meeting between the Sinn Fein leader and a member of the British royal family.
August 12, 2015 - Kevin McGuigan, an alleged former IRA member, is gunned down in Belfast. The lead investigator in the case calls McGuigan's death a "revenge killing." He says police believe the shooters were members of an organization called Action Against Drugs, which includes former members of the Provisional IRA. The Police Service of Northern Ireland's Chief Constable later says there is "no information to suggest that violence...was sanctioned or directed by senior level of the Republican movement."
August 26, 2015 - The Ulster Unionist Party announces its intent to resign from Northern Ireland's power sharing executive and form an opposition government. The Unionists group questions whether Republicans in the government could be trusted, if members of the Provisional IRA were involved with the murder of McGuigan.
September 9, 2015 - Three senior Irish Republican party members are arrested and released without being charged in connection with McGuigan's murder.
September 10, 2015 - As police intensify their investigations into the deaths of Davison and McGuigan, First Minister Peter Robinson of the Democratic Unionist Party steps aside. All but one of his party's other ministers resign, creating tumult within the power-sharing government.
September 29, 2015 - A year after Sinn Fein leader Gerry Adams is questioned, Northern Ireland's Public Prosecution Service announces that Adams and six other people will not be prosecuted in the 1972 abduction and death of Jean McConville, a Belfast resident and widowed mother of 10 who was reportedly killed by the IRA because the group believed she was a spy for the British army.