Assumption 1: America does not strike.
The extremists groups in the Middle East are already convinced (right or wrong) that America has weakened its resolve and the current President is a weak one. After drawing a red line on the use of chemical weapons, failure to act in the face of convincing evidence would erode what little is left of American influence in many of the Middle Eastern countries. Russia, waiting on the wings, is looking forward to fill in the vacuum. Iran will look at American failure to act as a sign that America no longer has the strength to enforce its policies, and Iran will therefore see it as a blank check to continue its development of nuclear weapons and its attempts to subvert neighboring Arab countries in its ongoing fight for supremacy in the Muslim world. The failure to react to the use of chemical weapons will also encourage all groups in the Middle East to go for it...if using banned weapons carry no consequences, why avoid them?. Picture Hezbollah or Al Qaeda with access to nerve agents and the will to use them.
Consequences for Israel? In the immediate time, no consequences. Down the road, Israel will face chemical terrorism and maybe even chemical weapons raining in missiles from Hezbollah-held southern Lebanon. The collapse of American credibility in the Middle East will also encourage Israel's enemies to take bolder action against the Jewish State; it will also stiffen Palestinian demands and unwillingness to compromise – not to mention the possibility of becoming unwillingly drawn into the Syrian quagmire.
Assumption 2: UN-supported limited action against Syria
The validation of the strike by the United Nations would add to the message of a limited strike. It would also act as a limited fix of America's image and credibility with the regional governments. It would weaken Assad's forces and would therefore weaken the strategic presence of Iran in the region. It would send Iran the message that the world means business and that failure to listen to the international community does carry consequences would, however, embolden the extremists elements which have made their way to Syria – groups which are mostly Sunni and supported by Bahrain and Saudi Arabia; some of these groups are loosely affiliated with the Al-Qaeda network. The international validation of the action, however, might give the Arab sponsors of these groups some pause, leaving some of these extremist groups without patrons and fending on their own.
Consequences for Israel? In the short term, it is likely that Israel will see an increase in attacks from Lebanon and from across the Golan border to force it to take sides and draw her into the Syrian civil war which could work to Iran's advantage allowing it to strengthen its presence in the Fertile Crescent. If Israel is not drawn into the Syrian swamp, however, the long term consequences of a UN sanctioned limited strike would probably be little if any. Unfortunately, it would also mean that the civil war will continue in a protracted mutual butcher bill, pushing more refugees into neighboring Arab countries and possibly destabilizing them – particularly Jordan.
Assumption 3: UN-sanctioned all-out strike against Assad's forces
Again, while international sanction would make the military action more palatable and would probably spread around the burden and the cost of such action – an all out strike against government forces in Syria might change the balance of power in favor of the insurgency, which is falling increasingly under the spell of the extremists. In the current situation, a rebel victory would not result in a democratic government but in another democratically-elected Islamist dictatorship and would take Syria down the same road negotiated by Egypt over the past two years with an unpredictable outcome. The Humanitarian consequences would be staggering both, in blood as well as in the misery of the survivors and the squalor of the refugee camps. Many refugees might choose to stay out of Syria rather than go back to an Islamist-ran Syria, increasing economic and social pressure on the host countries – possibly destabilizing Jordan. The Syrian refugees in Lebanon might even be forcibly returned to Syria in an attempt to destabilized the new regime. Syria might, in short, become a failed State in the heart of the Middle East.
Consequences for Israel? Israel would face growing and unrelenting terrorist activity from Syria. Since probably Damascus would have little effective control of the territory, it is likely that full training camps for Al-Qaeda affiliated groups will mushroom in a post-civil war Syria. It is also possible that Iran, in an attempt to recover standing in the Western Middle East, would go back to court Hamas. Since Hamas is now adrift after loosing the backing of the Brotherhood-led Egyptian government, it is possible that Hamas would welcome Iranian support. With a Sinai peninsula next door becoming no man's land with terrorist groups taking control, the prospects of a two front regional war is a likely scenario. How much of this war will spill into Israel? That is difficult to evaluate right now and it depends to a large extent on the actions of the Israeli government.
Assumption 4: US unilateral limited action against Syria
Much of what I wrote under Assumption 1 (UN-supported limited action) still holds, but the scenario gets more complicated. In case of US unilateral action, Russia might feel justified to support the other side. With Russian forces now being deployed in the Eastern Mediterranean to counterbalance the US Navy, the potential for a global war is high.
Assumption 5: US unilateral all-out strike
Many of the possibilities delineated in Assumption 3 are still valid – but again they get complicated by Russian intervention and maybe even Iranian. While Iran might choose to ignore a unilateral limited strike, it could not do so without loosing face in case of an all-out unilateral strike. Iranian involvement all but ensures a global war. Chinese dependence on Iranian oil might force them to take sides and/or use its leverage on the American economy. Any other possible scenario under this assumption gets worse. A US unilateral all-out strike, in my opinion, all but guarantees World War III including the possibility of a nuclear conflict.
None of the assumptions I used appears to lead to a happy ending. In any of these scenarios, the highest blood price will be paid by the Syrians themselves, but all the people in the region (including Israel) and around the globe could pay a high price as well. It is what Star Trek fans call a “Kobayashu Maru” scenario – damned if you do, damned if you don't. But in the Star Trek universe, James Kirk beat the scenario by changing the rules...and maybe the same could apply to Syria.
An internationally sponsored mediation between the government forces and the rebels could help defuse the situation. Such mediation would face incredible odds, but it seems like the only alternative that doesn't lead to more loss of innocent lives.
The conflict in Syria is between an Alawite (Shiite related) regime supported by the Shiite regime of Iran and rebel forces which are increasingly coming under control of Sunnite extremist groups (some of them Al-Qaeda affiliated) with support of Saudi Arabia and some of the Gulf Emirates. It is part of a wider conflict between Shiites and Sunnites which started in the first century of the Muslim era (VII Century CE) and continued unabated ever since. It is a conflict rooted in religious beliefs in a society in which religion and politics are very often intertwined if not blurred into each other. No amount of mediation can eliminate that centuries old enmity, but mediation could help the parties find a balancing point, a situation which would buy time. Maybe even a partition of Syria that would give the Alawites its own state while allowing the Sunnite majority to rule itself. This would probably trigger a partition of Iraq as well, but it would defuse an explosive situation which is threatening to drag the rest of the world into it.
I cannot help but remember a comment by Albert Einstein:
“I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones.”