OPEC often has difficulty agreeing on policy decisions because its member countries differ widely in their oil export capacities, production costs, reserves, geological features, population, economic development, budgetary situations, and political circumstances. Indeed, over the course of market cycles, oil reserves can themselves become a source of serious conflict, instability and imbalances, in what economists call the "natural resource curse". A further complication is that religion-linked conflicts in the Middle East are recurring features of the geopolitical landscape for this oil-rich region. Internationally important conflicts in OPEC's history have included the Six-Day War (1967), Yom Kippur War (1973), a hostage siege directed by Palestinian militants (1975), the Iranian Revolution (1979), Iran–Iraq War (1980–1988), Iraqi occupation of Kuwait (1990–1991), September 11 attacks by mostly Saudi hijackers (2001), American occupation of Iraq (2003–2011), Conflict in the Niger Delta (2004–present), Arab Spring (2010–2012), Libyan Crisis (2011–present), and international Embargo against Iran (2012–2016). Although events such as these can temporarily disrupt oil supplies and elevate prices, the frequent disputes and instabilities tend to limit OPEC's long-term cohesion and effectiveness.