Twenty-one months after the conflict in Syria began as a popular uprising, rebel forces are making gains, tactics are changing, and the threat of chemical warfare has made an appearance. Syrian rebels reached a level of cooperation, forming a single entity -- the Syrian National Coalition. The alliance has received recognition from Arab states and support from NATO members in its goal of unseating Syria's President, Bashar al-Assad, and replacing his government. But U.S. intelligence reports have noted activity within Syrian government-controlled chemical weapons facilities, and President Barack Obama has warned that the use of such weapons against rebels would cross a "red line." There are signs that al-Assad's hold on power is slipping as rebels gain ground and support, and even Russia, a longtime ally, has reportedly sent ships to the Syrian coast for a possible evacuation of Russian citizens. Collected here are images of this bloody conflict from just the past few weeks.