A conflict raging in Sudan is rattling neighbouring countries and worrying the United States and others for reasons ranging from concern about shared Nile waters and oil pipelines to the shape of a new government and a new humanitarian crisis in the making. Sudan, which relies heavily on foreign aid, is no stranger to conflict. But this time fighting is tearing apart the capital instead of a remote corner of a nation, which lies in an unstable region bordering the Red Sea, Sahel and Horn of Africa. Five of Sudan's seven neighbours - Ethiopia, Chad, Central African Republic, Libya and South Sudan - have faced political…