The death last May of Umaru Yar’Adua, Nigeria’s president, upended the informal agreement between members of the ruling People’s Democratic Party (PDP) to rotate power between northern Muslims and their southern, mainly Christian counterparts. Yar’Adua’s deputy, Goodluck Jonathan, from the oil-rich Niger Delta in the south, overcame resistance from members of the late president’s cabinet and was sworn in as Yar’Adua’s successor, as stipulated by the constitution.