The United States is building a coalition against Syria to respond to an alleged chemical weapons attack on April 7 against civilians and rebel forces in Douma, near Damascus. The primary objective of an operation against Syria will be to deter the further use of chemical weapons, something that a punitive missile strike launched last April by the United States did only temporarily. This time around, however, a U.S.-led strike against Syria likely would be wider in scope, aiming not only to deter Damascus, but also to impair the Syrian government's ability to carry out chemical weapons attacks. But even with the support of a coalition, and even with a bigger operational scope, the same factors that constrained the United States in April 2017 would also constrain a military operation this time. There are, however, some important differences between last year and now.