It is the world's longest river. A 7,000km life-line for almost 400 million people. It runs through 11 countries, including South Sudan, from the highlands in the heart of Africa to the shores of the Mediterranean Sea. It is a source of sustenance, but also of tension - and even potential conflict. In this three-part series, Al Jazeera examines the historical roots and present-day realities of the Struggle Over the Nile.