Sectarian religious conflict has contributed to some of the most intractable, bitter and painful episodes of aggression and persecution throughout history. For instance, conflict between Irish Catholics and Protestants has existed for centuries, bound closely with nationalist identities, playing out globally in North American cities, as well as the streets of Belfast and Glasgow. Another faultline where sectarian conflict has contribute to wider conflicts has been within Islam. Certain Sunni sects inspired by extreme Wahhabism and other ideologies have declared Shi’as (and sometimes mainstream Sunnis) to be heretics and/or apostates.
Longstanding and emerging divisions within groups, particularly within religions, could become an axis across which violence and conflict are articulated. Divisions within confessional groups could have destabilising effects within a region and around the world. Recent examples of this are Lebanon (where both sectarian and inter-confessional conflict co-exist) and Pakistan.
Sectarian conflicts are complicated and cannot be separated from geographical, cultural, political and economic contexts. For instance, sectarian divisions could influence, or be influenced by, how people experience and react to global economic competition and domestic economic frustrations (for example, high unemployment among Saudi and Iranian graduates). Economic frustrations could deepen and exacerbate sectarian divisions, influencing the outbreak of violence and conflict. These factors lead us towards the possible “wildcard” of a major eruption of sectarian conflict in highly sensitive regions of the world such as Iraq, Pakistan or Northern Ireland, discussed in more detail below.
Iraq: Though complicated by ethnicity, social classes and social identities, sectarianism between Sunni and Shi’a Muslims, compounded by conflict with the Kurdish population, could influence future political stability in Iraq. Ethno-sectarian dimensions have shaped possible plans for future government structures in Iraq, such as the Biden-Gelb and Brookings Plans, which suggest varying forms of decentralising or partitioning Iraq along Sunni, Shi’a and Kurdish lines. With an American withdrawal, sectarianism is likely to strongly influence the evolving political, economic and social landscape in the country. Sectarian divisions could be an axis along which conflict develops between mainstream political parties. For instance, events to increase perceived marginalisation or isolation by one group, such as Kurdish marginalisation through a Sunni-Shi’a coalition, might incite violent conflict and political instability.  
Pakistan: Conflict in Pakistan has strategic, territorial and ethnic dimensions. Islamic sectarianism in Pakistan could be fuelled by ongoing violence and conflicts in the Waziristan region that joins Afghanistan and Pakistan, which has a strong Taliban presence. Strong independence sentiments could provide a basis for religious militants and extremism. Divisions between Muslims (e.g. between mainstream and more extremist interpretations) could influence how Pakistanis react to militants and extremism. Reactions could be complicated through international politics, for instance pressure from the United States (US) to control militants could shape internal divisions between Muslims. 
Northern Ireland: Sectarian religious divisions in Northern Ireland are closely bound up with republican and loyalist divisions. In recent years, international support and the resource base for republican dissidents appears to have subsided. As well, dissidents and loyalists in Northern Ireland have decommissioned weapons. ...