If you'd like to read a fairly short but very insightful item to gain a better understanding of the strategic context for much of the conflict in the Middle East read this item Stephen Crittenden, The Clash Within Civilisations: How The Sunni-Shiite Divide Cleaves The Middle East. From the opening:
"It is almost 20 years since the late Professor Samuel Huntington published his famous Foreign Affairs article, "The Clash of Civilizations?", arguing that cultural and religious differences would be the major source of conflict in the post-Cold War era. Forecasting a looming clash between Islam and the West, and another between China and the West, he wrote: "The fault lines between civilizations will be the battle lines of the future."
"But what about the fault lines that emerge within civilisations, and especially within the religious traditions those civilisations are founded upon? There is a dangerous 2,000-kilometre fault line running through the Middle East between Beirut and Bahrain via Damascus and Baghdad, which marks the present line of demarcation between the two main branches of Islam, Sunni and Shiite.
"The 1,300-year-old schism between Sunnis and Shiites was caused not by a theological dispute (those came later), but by rival clans in Muhammad's tribe, the Quraysh, squabbling over the succession after his death in 632 AD.
"Mostly the "Sunni-Shia Line" lies dormant, and ordinary Sunnis and Shiites live out their separate lives, side-by-side in relative harmony. In Lebanon and Iraq it has not been uncommon for Sunnis and Shiites to intermarry. But the Line is still always there, just below the surface, and it has recently re-emerged as the most significant factor reshaping geopolitical relationships in the Middle East, a region where religion and politics are always inextricably intertwined.
"At present, Syria is the key battleground on the Sunni-Shia Line..."
As I have said on many occasions the larger contest with which the U.S. should be most concerned is the strategic competition between Sunni and Shia and between Arabs and Persians. This competition is made manifest in the struggle for power and influence within Islam between Saudi Arabia and Iran. To the extent the U.S. involves itself in the Middle East and chooses various actors to support, it should do so with its eye firmly fixed on this competition.