the spark is, the overthrow of the Sunni ...or attempt of it by the Shia...Its already come close....look at the spark that caused Egypt and Tunisia....one guy in protest setting himself on fire...could be anything like that...why it spins to world war..is pretty simple..oil interests....nations will back sides...and things will spiral off from there as more powerful nations move to take over the many weak nations of the middle east to protect there flow of oil...Iran, Saudis, Egypt, UAE, Russia, USA< Syria, Lebanon, Turkeyand Europe will not just sit back and watch and hope..as the worlds oil reserves get carved up.
I'm not sure I buy into this argument. We are already seeing all these conflicts (Egypt, Libya, Tunisia, Yemen, Bahrain etc etc) and I think the idea we (or anyone else really) would invade those nations is quite a long shot.