Syrian's military has won a key victory against rebel forces, by driving them out of the city of Qusair, an important outpost for both sides if they hope to control the rest of the country. Qusair not only connects to a key border crossing with Lebanon, but the town sits just off the main north-south highway that connects Syria's three major cities — Damascus, Homs, and Aleppo — and near the crossroad that connects Homs to the Mediterranean. (If you think of Syria's population centers as an actual cross on a map, Homs is the intersection, and Quasir is just a bit off-center from that.) Controlling that strategic hub was a key to disrupting the government's grip on the whole country. The loss of it, could lead to a serious setback for the scattered rebel armies.
It also appears that the government victory was only made possible by the support of Hezbollah fighters who came over the border from Lebanon to assist in the offensive. As many others have written, this may be a key victory for them, but it is a strategic risk for the Iranian backed militant group. They have always styled themselves as a resistance group fighting back against Israel, but going on the offensive puts them at risk of losing more moderate supporters and could give the Israelis an excuse to intervene. (The Israelis claimed that earlier attacks inside Syria were not an assault on Bashar al-Assad, but were merely to prevent arms from being shipped to Lebanon.) Hezbollah's involvement makes the war a more explicitly sectarian conflict and that makes it even more brutal and harder to end.
The return of Qusair to government control also comes as Russian and U.S. officials meet to discuss the conflict with the hopes of a settlement dimming. With the tide turning in favor of Assad, there would seem to be even less reason for his allies to back down and look for a negotiated peace.
On the other hand, people are already wondering how the promotion of Susan Rice and Samantha Power (who won a Pulitzer for her writings about genocide and human rights atrocities) to new national security posts could alter the U.S.'s mostly impotent (so far) policy toward the war. Both have a reputation for supporting intervention — like they did in Libya, and wish they could have in Rwanda — but are also realistic enough to know that it the Syrian case it might do more harm than good.