One of the problem when a western looks at the mideast is that the West has NOT been tribal since the days of the Roman Empire. During the "Darks Ages" Community more than one's Tribe was how people identify oneself (and this was expressed in terms of religion, thus the big hold of Christianity during both the Dark ages and the subsequent Middle Ages). While the concept of Nation-State seems to start before the Reformation and Renaissance, it continued to developed during the Enlightenment and along with Nationalism became the dominate way people saw themselves starting about the time of the French Revolution. Thus westerns tend to be identify themselves as being a member of whatever nation they are in more than anything else (In many ways Communism was a rejection of this development of the Nation-State going back to the Community concepts of the Middle; Ages, but again a rejection of Tribalism).
On the other hand Tribalism is alive and while in the Middle East. In the 1980s the Shiite Muslims were about to win the Lebanese Civil War, so the Christan Maronite turned to the Alawite of Syria to bale them out (These two Tribes had been allied for Centuries and shared trade routes with each other). Recently these two had had a falling out, with the Syrians backing Hezbollah (Who get support from some Christian Tribes).
In Iraq and Arabia tribal identity is the most important thing for a person, more so than Religion. Religion is often tied in with one tribe or another and often identify long times allies (Shiites tend to ally with Shiites, Sunnis with Sunnis, Christians with Christians), but alliances across religion also exist. Tribal lines often cross national lines (For example the Alawites of Syria and their Alliance with the Christians of Lebanon), sometime national line split Tribes. In the case of the latter the tribe is really not loyal to either country his tribe is in, the tribe is loyal to itself and its members. Tribes can also co-exist in the same area mostly for trade but also where tribal groups overlap and it is easier to live together than to fight all the time.
Once you understand how the tribes work, then what is happening in Iraq and Arabia begins to make sense. The US invaded Iraq to control Iraqi oil, the Sunnis, who under Saddam, ruled Iraq revolted and called in support from their fellow tribal members both in Iraq and Arabia. These "Foreign" Fighters brought in with them some of their religious friends in addition to fellow tribe members (the Religious fanatics are bin Laden contribution to this mess). While the religious fanatics are a factor the big factor are these tribal connection. Not only are fighters coming over from tribal members living in Arabia, those tribal members are using their connections in Arabia to raise money and arms for they fellow tribal members in Iraq. It is these tribal leaders who, I believe, have been supplying the arms to the Sunni insurgents in Iraq. These tribal leaders (both in tribes in both arabia and Iraq but also tribes allied with other tribes in Iraq) have been sending in the arms and dragging the House of Saud into Iraq.
Now that the some tribal members are in Iraq, the House of Saud must "support" them to a degree. Like Saddam, the House of Saud depend to a very large degree on the support of the tribal chiefs of the Mid-east. If the House of Saud should lose support from these Chiefs, the rule of the House of Saud is finished. The same went for Saddam so he took care of the Tribal leaders (Who are elected by the male members of the tribe whenever one dies, thus while Saddam could kill off the tribal Chief he could NOT select his successor, the same goes for the House of Saud).
Religion comes into play in that most Tribe identify themselves not only by blood ties (Cousin marrying is the norm in the Middle East thus you have VERY close families) but also religion. The Shiites identify tribe with historical Strong ties with Iran. Christian Tribes are tied in with various trade routes in the area. Sunni tied in with Baghdad and the tribes tied in with ruling from Baghdad. In many ways the Shiite- Sunni split almost follows the Roman-Persian split of the about 100 BC to 600 BC. which in turn reflected how far each army could go say from they base of support (The Mediterranean for the Romans, the Persian highlands for the Persians).
Thus the split along religious lines is NOT the cause of this conflict, it is only one of the many ways the tribes identify themselves and who they are allied with. The Saudis are tied in with the Wahabi, but most of Arabia is not (Being Sunni instead, Wahabi is just a radical offshoot of Suunism). The Shiites control Persia, southeast Iraq as while as BOTH SIDES of the Persian Gulf (The States of the Persian Gulf are lead by Sunnis, but the people are Shiites, thus the leadership tend to be a very strict dictatorships, a tiny Sunni elite ruling over a large and hostile Shiite majority.
In many ways the Shiite Success in Iraq is more feared then any US withdraw, for the Shiites show the other Shiites of the Gulf that you to can RULE. All of these groups have string ties with Iran (Shown by all of them being Shiites).
One last Comment, when it comes to Secular rule, the Shiites have preferred it much more than the Sunnis have. Sunnism has in its world view that the Ruler is the agent of God and thus he must Rule in God's name and matter. The Shiites have always been the minority in Islam and as such often under hostile rule by Sunnis. Thus the Shiite developed a much stronger concept of Separation of Church and State than did the Sunnis, for the Shiites were more often than not forced to live under the rule of a Sunni Ruler. Modern Iran is an exception to this rule, but even in Iran an attempt was made to keep religion and government Separate while making the Government subservient to the religious establishment. Thus you have an elected Parliament and President, but who may run is determined by the religious authorities. Even some Shiite are uncomfortable with the setup, but some sort of religious control was going to be imposed after the Shah, but popular will was also to be given a voice. The present Iranian Constitution is a compromise between there two concept, one we in the West can NOT fully agree with, but one that is understandable given the previous rule of the Shah.
More on Sunni Islam:
http://en.wikipedia.org/wiki/SunniMore on Wahabi:
http://en.wikipedia.org/wiki/WahabiMore on Shiite Islam:
http://en.wikipedia.org/wiki/Shi%27a_IslamChristianity in Lebanon:
http://en.wikipedia.org/wiki/Christianity_in_LebanonMaronite Christians:
http://en.wikipedia.org/wiki/MaroniteFor more on the Alawites:
http://www.danielpipes.org/article/191http://en.wikipedia.org/wiki/Alawite