As the government in Syria continues to lose ground the problem of their large arsenal of chemical weapons is growing. Should weapons fall into the hands of unsavory characters that happen to hate American one can see the danger to American interests. The U.S. is planning for contingencies that I think will result in a large scale operation to secure, remove, and destroy weapons depots. This would not likely be a long-term situation but it will be very difficult.
If they don't:
If they don't: