Our Expert Voices conversation on what comes next in Iran vs. Saudi Arabia.

The passing of Iran's supreme leader and the Saudi king represents a pair of known unknowns: leadership transitions that are long anticipated and carefully planned, but whose realization will provide rare openings for jockeying by domestic rivals, testing by external adversaries, and possibly even mobilization on the streets. Each ruling system is likely to survive, but only if leaders can manage an array of new challenges such as the sense of political entitlement among Iran's post-revolutionary generation, the fissures within the Saudi royal family, and the structural distortions that plague both economies.

The bottom line: The strategic competition and sectarian tensions between Riyadh and Tehran will almost certainly outlive the senior leadership of the two theocracies. Both engage in pan-Islamic posturing, but sectarian absolutism is baked into the ideology and institutions of Iran's Islamic Republic and the Saudi monarchy. New leadership won't be inclined or empowered to rise above it.

Other voices in the conversation:

• Ali Shihabi, Saudi Arabia policy analyst, Arabia Foundation: Succession in Iran will escalate tensions
• Bernard Haykel, Middle East scholar, Princeton University: Don't expect de-escalation
• Dalia Dassa Kaye, political scientist, RAND: Youth will drive change
• Karim Sadjadpour, Middle East policy analyst, CEIP: The rivalry will grow