1. the Middle East remains a strategic center of gravity in the world for two major reasons: the oil and the shipping chokepoints, and huge chunks of the world economy are dependent on both of those. instability in the region threatens those two facets, thus threatening the world (and our) economy.
2. the Middle East is inherently unstable, as demonstrated by nothing better than recent events. Tyrannical governments keep their populace in line with the stick of the mukhaberat nd the carrot of the welfare state based on revenues generated from nationalized resources (read: oil and the Suez). But that rentier state carrot is intensely vulnerable to falling revenues and - as the Iranian Shah and Mubarak learned to their chagrin - can rapidly inspire revolution followed by replacement by radical (and themselves inherently destabilizing) elements. Internally, the Middle East is a bubbling cauldron, and the resources upon which much of the worlds' economy is based right there in the middle.
Internationally, among the Sunnis, Egypt and Saudi Arabia both consider themselves the natural leaders, and have already proven willing in Yemen to shoot at each other over that disagreement. The Iraqi's also consider themselves the natural leader of the Arab world, but lately they haven't been a serious contender. The Saudis are currently attempting to take control over the region through the exportation of Wahabism, which is itself inherently destabilizing, as it preaches the overthrow of the National-Socialist model governments left over from the 60's and 70's in Egypt (check) and Pakistan, (as well, obviously, as the democracy - as much as it exists - in Lebanon and in Israel) followed by the violent unification of the region under a single banner, followed by an invasion of the rest of the world. They aren't kidding about that part, and we are idiots if we fail to take them at their word, especially as they seem to have just succeeded in part A of step 1, the removal of the Mubarak regime.
The Iranians are the largest terror-exporting nation in the world, and they are very, very good at it. The IRGC, and in particular the Quds forces, have fostered the growth of Hezbollah (the real deadliest terrorist network in the world - Al Quada was their student, not the other way around), Hamas, and even (through proxies) Al Quada. They are currently waging a campaign to destroy the Lebanese government, and are strengthening ties with Syria and Turkey in an attempt to build a base with which to challenge the US and Saudi Arabia for dominance of the region, part of that struggle (they assume) including the destruction of Israel. The leadership of that nation Really Believes that the 13th Imam is coming soon, and that they must kick off international Jihad in order for him to arrive and bring about the End Times - and again, we are fools if we fail to take them at their word on that.
3. the region, thus, needs an overpowering, hegemon if it is to remain stable enough to ensure the non-collapse of the world economy. Someone has to impose order and keep these nutjobs from destroying the ability of the world to access the oil and the suez. There is only one nation currently on the planet with the capacity to perform this task: the US. The US Fifth Fleet, currently headquartered in Bahrain, is the major (and perhaps only realistic) force for stability in that region, contending with numerous, powerful forces for instability.
4. Withdrawal or severe downdrawl of US Forces would create a power vacuum and kick off fights within the sunni community and between Iran and Saudi Arabia for regional dominance. Shiite Iran is seeking to get nukes. Syria has had a nuclear facility already destroyed by the Israelis. Sunni Pakistan (see: Wahhabi plans for governments, the overthrow and replacement of) already has them. In the face of a US Withdrawal, Saudi Arabia certainly would start developing her own.
Imagine a Mexican standoff, except that 3 of the 4 players are A) paranoid schizophrenics facing opponents they violently hate, B) convinced that death will be a net benefit for them, C) convinced that their souls are in peril if they don't shoot, and D) potentially armed with nukes (the 4th Player is the unfortunately-located Israel). I think everyone here can agree that that is not a "stable" situation, particularly when you add in E) these countries are not internally stable, but may feel forced into an external war in order to solidify internal support and F) at least two of the players (Iran and Saudi Arabia) are held hostage by their own extremists, who feel free to act without permission, are nearly impossible to stop, and are most desirous of the conflict. And I feel that A) deserves rementioning.
FUN FACTS WORTH NOTING: China (also nuclear) is rapidly becoming a good, good friend of Iran, and is semi-distancing itself from Pakistan (whom it largely views as a foil against India). China is also heavily invested in East Africa. It is possible that China would seek to intervene in the region to tilt the balance in Iran's favor as the US did in Saudi Arabia's. If that happens, then the newly Taliban (and nuclear!) Pakistan - which is deeply paranoid, xenophobic, and a wierd mixture of Wahhabist and neo-Deobandi - becomes an ally of Saudi Arabia, and our players are all now holding two pistols even as their inner demons scream at them to shoot first. BEST CASE SCENARIO here is that China is able to stabilize (kinda) the region, and merely takes all the oil for itself - only partially collapsing the world economy. but that's the "best" case, not the "most likely" one. it's not even really a "sorta likely" or a "semi likely" one...
