Tribal and trade politics are also factors. I once read that Syria only entered into Lebanon when the leading trading partner of the Alawites came under attack. I forget who was doing the attacking (I think it was Sunni Moslems) but the President of Syria defended his tribe's long time ally and trading partners, a CHRISTIAN Tribe.
Sometime it is easy to follow the tangled mess of the Middle East, but that is rare. Tribal politics have often overcame Religion, nationality and even trading partnerships. Saddam, for example, before the recent war contacted the head of all of the Tribes of Iraq to secure their support or at least to get them NOT to support the Americans. Some of these tribes are Sunni, others are Shiia at least one "worships" Lucifer (Through that particular tribe, the Yezidis, believes God forgave Lucifer for his uprising and restored Lucifer to his place as God's chief Angel).
Iraq has over 150 tribes. While roughly 1/4 of its populations do not belong to any tribe (At least on paper) many of these go through their local tribal system for assistance. This is a situation throughout the Middle East. One's tribe is often more important than one's Religion (and always more important than one's country). As stated above, Syria's Ruling Alawite had the Syria Army enter Lebanon to protect the Christian tribe that had century long trading ties with the Alawites.
My point here is while Religion is important, Tribe and Trade relations are also important. While it is easier to trade within one's tribe that is not always possible, in such cases you go with traditional trading partners. Often these tend to be the same religion but not always. The ins and outs of the Middle East since the Arab Conquest have complicated the problem.
Now you must keep in mind the basic geography of the the Middle East. The most important Rivers are the Tigris and Euphrates, both in modern Iraq, but their headwaters are in the Mountains of Syria, Lebanon Turkey AND IRAN. The largest three Countries in the Middle East is Iran, Egypt and Turkey. Over the last three hundred years these countries have fought each other and whatever Country Iraq was formed into over and around the Middle East. The various Tribes of the Middle East has use these divisions to their own benefit.
Of the Three major power (Egypt, Turkey and Iran) the Country with the hardest time to get into the Middle East Proper is Egypt (Palestine is a long hard March to the headwater of the Euphrates). While Egypt has the hardest March, it has the fewest problems with other fronts (Libya has rarely been strong enough to threaten Egypt and Sudan can be blocks at several points on the Nile so also rarely a threat to Egypt and while Egypt can be conquered from the Mediterranean, once ashore the invaders have rarely succeeded do restrictions as how many people you can ship and the population of Egypt, thus the Various Crusades against Egypt failed, Napoleon sea attack failed, when Egypt has been conquered it has been from an attack from Palestine, through at least one Libyans and an Nubian Invasion were successful).
Unlike Egypt, Turkey is subject to invasion from Russia across the Black Sea, from Europe through the Balkans, from the Mediterranean, and from the East. In fact the Greeks lost what is now Turkey to the Turks over a 1000 years attack and settlement of what is now Turkey by the Turks. Iran is not as bad as Turkey when it comes to being attacked but it still has had to protect itself from invasions from Southern Russia (Including the attacks of the Seljuk Turks and the Mongols) and has been involved in right is now Pakistan since the time of Cyrus the Great (529BC).
Thus these three Countries (and you have to replace Turkey with Greece if you are talking about dates prior to the Crusades) have ebbed and flowed throughout the middle east for at least 3000 years. This leads to some strange alliances between tribes of the Middle East. Cyrus the Great was a follower of Zoroastrianism and as such the Persian Empire from his reign till the Arab Conquest always supported Zoroastrianism in the Middle East. In fact the modern split between Sunni and Shiite Islam follows closely the border between the Roman Empire and the Persian Empire (The Border roughly stayed the Same from about 100 BC Till 600 AD). With the Arab Conquest (c622 AD) the Middle East came under Arab influence but Greek influence stayed strong given the control of the Mediterranean by the Greeks even after the lost of Egypt and Syria.
To show you that religion can be politics lets look at he last big split in the Eastern Church which was on the nature of Christ, did he have one nature or two natures? This was referred to as "Monophysitism" heresy. This was debated for almost 200 years after the fall of the Roman Empire in the West. On the surface it looks like a minor religious dispute, but if you look at who advocated what you see that the dispute was important NOT for the dispute but who was doing the disputing. After the fall of the Roman Empire in the West, the Eastern Empire lasted another 1000 years, but the 200 years between the Fall of the Western Empire and the Arab Conquest saw a growing dispute between Greece and Egypt. While the Roman Empire in the West existed these two Countries could balance each other and the West, but once the West had fallen these two Countries started to argue over who should be ruling the Eastern Empire. The religious dispute was just how the split manifested itself, once the Arabs took over Egypt and Syria the dispute died away (It technically still exists, the Christian Churches of Egypt and Syria follows one teaching the Greeks follow the other, but neither view it as something to fight over, something their ancestor around 500-600 AD would have disagreed with). My point here is that the religious dispute was just how the dispute between Egypt and Greece manifested itself, the dispute really was NOT over religion but who was to rule whom.
The reason I bring out this point is often the split in religion reflect political infighting within the Middle East. The split caused by Monophysitism reflected more on the growing tension between Egypt and Greece (with the fall of the West) than a real religious break. Many of the other splits in both the Christian and Islamic Religions reflect similar splits. I pointed out the split between Shiia and Sunni in Iraq approximates the old Roman-Persian Border (and the split may just be the result of how people and goods move in these area than a real religious split). One of the great Split in Islam took almost 400 years to work out. At the Death of Ali, the Shiia maintain the Caliphate should have stayed in his family, but the Majority of the followers of Islam disagreed and help establish the Umayyad Dynasty (661-750AD). This Dynasty took the Arab Conquest to its height but it was a shadow conquest, a mere over-lordship that like other such empires fell within 100 years of its height. It could only survived as long as it expanded and soon as it could no longer expand it collapsed into itself. Beside spreading the word of Islam to the World it provided no long term effect on world history. As it fell, The Eastern Roman empire planned its reconquest of Syria and Egypt, Greece launched several invasions during this collapse but never quite got around to taking it all back.
As the Umayyad Empire Collapsed, descendent's of Mohammad father in Law decided to take over what was left of the Empire. Given the discrimination Arabs had to non-Arabic followers of Islam these non-Arabic Moslem's help the Abbasids (750-1258) get into power. While the Abbasids never were able to expand the area of their Empire, they made several contributions to the situation in the middle east.
At first the Abbasids seem to have been allied with the Shiia, some sources call them Shiia, but over the next 100 years the Abbasid slowly move away from Shiism and develop modern Sunni version of Islam. While this change was occurring Shiites were grabbing power in Egypt and other areas in the Palestine and Lebanon. These were all replaced just before the Crusades with Sunni rulers. While the leadership of the Countries were replaced many of their followers stayed around and just hide out in the mountains.
This was further complicated by the Shiia breaking into two large factions themselves, the "Seveners" and the "Twelvers" named after how many Imams their recognized. The "Seveners" recognize only 7 Imams and disagree with the Twelvers over the seventh Imam. The Twelvers recognize 12 imams and than claim that the Twelfth will come back. Most Shiia today are Twelvers, but the Assassins were Seveners. This split still exists in the Middle East and reflects splits in tribes as while as religion.
The Crusades brought back some Christianity to the Region but mostly the Crusaders just protected the Christian already in Palestine AND Moslems that could be used in fights against other Moslems (Mostly splits from other Moslem groups for example the Assassins against their fellow Seveners the Ismalis).
Thus you have several different Moslem and Christian religions in this region, some with strong Christian influence, some with Zoroastrian Influence, but all loyal to their tribe more than their religion.
The Caliphs:
http://en.wikipedia.org/wiki/Caliph#DynastiesSome more on the Yezidis:
http://www.frontpagemag.com/articles/ReadArticle.asp?ID=5128http://www.religionnewsblog.com/5213-.htmlhttp://www.freewebs.com/see_the_truth/Yezidis.htmlOn the Assassins:
http://ismaili.net/assasyria.htmlhttp://www.alamut.com/subj/ideologies/alamut/secDoctrines.htmlhttp://www.sunnahonline.com/ilm/seerah/0056.htmOn the Alawites:
http://www.muslimhope.com/alawites.htmhttp://open-encyclopedia.com/AlawitesWahhabism (Main Religion of Saudi Arabia)
http://i-cias.com/cgi-bin/eo-direct.pl?assad_hafiz.htmHospitallers of St. John of Jerusalem
http://www.newadvent.org/cathen/07477a.htmCyrus the Great:
http://en.wikipedia.org/wiki/Cyrus_the_Great