What's behind the recent violence in Northern Ireland?

For over a week, riots have marred the streets across five cities and towns in Northern Ireland. Cars and a bus have been hijacked and burned, young people have thrown petrol bombs at police, and at least 74 officers have been injured.

The escalating unrest threatens to undermine the region's fragile peace between pro-British loyalists who want to remain part of the United Kingdom, and pro-Irish nationalists who would like Northern Ireland to be part of the Republic of Ireland. When did the most recent flareup begin?