Relations between Sunnis and Shi'is, Islam's two main sects, have been tense for generations. Yet only lately have they become confrontational. Inter-confessional strife has increased, owing to the dynamic nature of politics in Muslim states during recent decades.

Revolution, civil war and regime change have galvanized Shi'is into direct political action, regionally and internationally. Conflict between the sects has further intensified in the fallout from the Arab uprisings that began in 2011, placing Shi'is at the vertex of developments in Bahrain, Yemen, Iraq, and Syria.

Yet Sunni - Shi'i relations have been marked by cooperation as well as conflict, and this dispassionate study of Shi'i politics in modern times argues that the relationship matters more to the Middle East now, and to the Muslim world, than at any time since the demise of the Ottoman Empire.

Anoushiravan Ehteshami is the Nasser al-Mohammad Al-Sabah Chair in International Relations and Director of the HH Sheikh Nasser al-Mohammad Al-Sabah Programme in International Relations, Regional Politics and Security, both at Durham University. He has been a Fellow of the World Economic Forum, and was elected in 2011 as a member of its foremost body, the Global Agenda Councils.

Dr. Mahjoob Zweiri is Associate Professor in Contemporary History and Politics of the Middle East and Head of the Humanities Department at Qatar University. He was a senior researcher in Middle East Politics and Iran at the Center for Strategic Studies, University of Jordan, and a Fellow of the School of Government and International Affairs, Durham University.