It is my understanding that these tensions didn't really exist as a significant force before the ethnic divide-and-conquer post invasion. after all, there aren't violent Sunni/Shia clashes in neighbouring countries every time a leader changes. I admit that there is a possibility of post-saddam ethnic tensions, just like i don't completely rule out civil unrest in Finnland, if an ethnic swede is elected, but I assign comparativly low probability to this happening.
Oh so the ethnic, inter-faith rivalries operating in Iraq are the fault of the Americans as well, are they? Nothing to do with the leaders of a small Sunni minority brutally repressing the Shia. Nothing to do with the deposed members of the Baath party, bitter at losing power, attacking the Shia with thet goal of provoking state failure and their return to dictatorial government. Nothing to do with the Shia, still suffering under Sunni death squads and worried that the Americans, so human and fallible after all, might sell them out to the Sunni / Baathi
again, forming militias of their own and fighting back, taking Iraq to the brink, perhaps beyond the brink, of civil war.
Also, "...every time a leader changes" is nonsense. There are few examples of ruling Sunni factions being deposed by Americans to install Shia dominated democratic governments. Most times leaders change in the Middle East it represents not the loss of power for one group, but the passing of a position between a small and limited network of people (Saudi Arabia, Sryia, etc), and in those instances where regimes have been toppled, replacing the ruling elite with a totally new group, I'm sure there has been plenty of bloodshed between the competing factions.