Saudi Arabia's execution of the Shiite cleric Sheikh Nimr al-Nimr early this month has stoked tensions between it and Iran. This political confrontation has been playing out across the region as Sunnis and Shiites wage proxy wars across the Middle East and Persian Gulf. This battle will intensify as domestic pressures mount in both Saudi Arabia and Iran, and regional geopolitics become more fluid.

Saudi Arabia, a Sunni kingdom, and Iran, a Shiite Islamic republic, have contested regional leadership for centuries, a struggle that was both political and religious. Until 1979, Iran was ascendant, a position that could be credited to Tehran's alliance with the United States. The Iranian Revolution ended that partnership, and Saudi Arabia quickly filled the gap as the closest U.S. partner among Arab states. Washington worked closely with Riyadh, and other regional governments, to contain Iranian influence.

The success of that effort prompted Tehran to sponsor proxies in such countries as Syria, Lebanon and Yemen, as well as to urge Shiite communities throughout the region to stand up against Sunni oppression. Conclusion of a nuclear agreement with the P5+1 heralds the end of Iran's isolation and renewed efforts to extend the country's influence in the Middle East and beyond.