A mission to destroy Syrian chemical weapons stocks could perform a preventive, preemptive or mitigating measure. Effectively degrading the entire arsenal would likely require an extremely wide target set. Syria has roughly 50 sites involved in manufacturing or storing chemical weapons  . Its arsenal consists of G and V-series nerve agents, which block neurotransmitters, causing convulsions and death through loss of respiratory control, as well as blistering agents, whose chemical burns restrict respiration and form large, painful blisters on the skin. Both are absorbable through the lungs or skin, requiring a full body suit for adequate protection. Between Syria’s VX, Sarin, and Tabun nerve agents, and its mustard gas blistering agents, this totals to several hundred tons of chemical agents stockpiled for combat use.
Ideally, a mission to destroy Syrian WMDs would focus on these stockpiles before regime forces forward deploy them to artillery or missile units, or before said stockpiles fall to rebel forces, which may lack the training, equipment, or discipline to safeguard chemical weapons for teams to inspect and dispose of them. Syria has more than 3,400 artillery pieces, many of which can fire chemically armed shells or rockets. Chemically armed artillery rounds would also be easiest to smuggle out of Syria. Syria also has significant surface-to-surface missile stocks, including roughly 250 range SS-21s and Iranian-made Fateh 110s, and up to 300 Scud missiles. 
While a preventive strike on Syrian WMDs might be able to destroy all large stockpiles, it would be extremely difficult for airstrikes to eliminate all Syrian ballistic missiles and warheads, let alone chemically armed artillery rounds. In the 1991 war with Iraq, the infamous Scud hunt required American and British Special Operations Forces (SOF), using helicopters and ground vehicles, to scour Iraq’s western desert for mobile sites. They destroyed relatively few missiles or mobile launchers, and there is no proof aerial interdiction from strike aircraft hit a single genuine Scud missile or launcher (Iraq maintained many decoy systems). Iraq retained a significant (albeit increasingly decrepit) Scud stockpile through the 2003 invasion. It is likely Syria would attempt to imitate Iraq’s success in dispersing and disguising its missile arsenal during any American bombing campaign, which would necessitate significant numbers of foreign SOF, tightly coordinating with strike aircraft that could operate with relative freedom from Syria’s Integrated Air Defense Systems. Without an extended aerial and naval strike campaign, supplemented by extensive Western SOF operations, a Syrian WMD hunt, even a preventive one, would be extremely difficult.
Even if the intervention did effectively service WMD targets, the risk of doing so under poor weather conditions, or in populated areas, would be serious. The U.S. knows this well from its own history, when German aircraft destroyed the SS John Harvey while it brought 60 tons of mustard gas–loaded artillery shells to southern Italy in 1943. Intended to respond to German chemical warfare, hundreds of soldiers fell victim to mustard gas symptoms. Given Syria’s large stockpiles, several of which are co-located with urban centers, any strike would need to carefully plan for favorable conditions to avoid causing civilian casualties. Not only that, but partially-destroyed and contaminated weapons might be abandoned, opening them to looting by third parties.
Given the obstacles to simply destroying Syria’s chemical weapons and associated launchers, the deterrent threat President Obama issued may seem more advisable. Since Obama left the deterrent threat ambiguous, U.S. military intervention in response could range from smaller-scale strikes on WMD-related and regime targets, as in 1998’s Operation Desert Fox against Iraq, to a full-scale air campaign to attempt to dismantle the Syrian regime in support of Free Syrian Army troops. Since Syria has only threatened to use its WMDs in the event of a foreign intervention, it will remain unclear for some time whether Obama’s threat was instrumental or a secondary concern in discouraging Syrian WMD use.
Yet a deterrence operation that sidesteps the problems of outright destroying Syria’s WMDs fails to address concerns about their potential proliferation into the hands of terrorists, foreign states, or Syrian combatants unaccountable to a chain of command the U.S. can target through deterrent threats. Securing Syrian WMD sites on the ground during a civil war would far outstrip any combination of allied SOF resources. While Joint Special Operations Command would still play a critical role, since they command some of the few units ready to conduct render safe operations under combat conditions, additional personnel, both military and civilian, operating with organizations such as the Army’s 20th Support Command and the Defense Threat Reduction Agency, would likely deploy later on. However, given the possibility of regime military resistance, or units affiliated with Iran’s Revolutionary Guards Corps and anti-American insurgents, attacking these units, large number of conventional troops would accompany these personnel as a force-protection measure.
Thus, the early estimates of 75,000 troops (along with, presumably, the establishment of air superiority and the execution of close-air support missions) participating in an effort to secure and render safe Syrian WMD are not unreasonable, though given the logistical constraints, any deployment would likely be under-resourced relative to the Pentagon’s preferences.
It would also likely be a long process. The U.S. and allies would need to secure transit, and then ship, chemical weapons to established facilities used in decommissioning their own chemical weapons stocks during the Cold War. That or wait for limited numbers of mobile sites and potentially hazardous controlled detonations to finish the job in country. In any case, efforts to fully destroy Syria’s WMD stockpiles could take years, which would also demand an extended force-protection mission by foreign troops or contractors.
Simply put, there are few easy options for destroying Syria’s WMD stockpiles if deterrent threats fail, and almost any strikes would need to follow on previously discussed suppression of enemy air defense efforts. Attacking Syria’s sites mainly from the air would require servicing 50 fixed targets and dozens or hundreds of mobile launchers and missiles, and require a sizable SOF presence inside of Syria to aid targeting. Even if successful, the risk of contamination and proliferation would remain, demanding thousands of foreign personnel inserting themselves into what would be, at best, the smoldering aftermath of a massive civil war. Though nonproliferation and deterring WMD use seems a simpler undertaking than protecting safe zones or overthrowing the Syrian government, it might also be the easiest way into a significant and open-ended Western presence in Syria, during and after its internal conflict.