A very helpful explanation from Max Fisher at The Washington Post. From the introduction:
The United States and allies are preparing for a possibly imminent series of limited military strikes against Syria, the first direct U.S. intervention in the two-year civil war, in retaliation for President Bashar al-Assad’s suspected use of chemical weapons against civilians.
If you found the above sentence kind of confusing, or aren’t exactly sure why Syria is fighting a civil war, or even where Syria is located, then this is the article for you. What’s happening in Syria is really important, but it can also be confusing and difficult to follow even for those of us glued to it.
Here, then, are the most basic answers to your most basic questions. First, a disclaimer: Syria and its history are really complicated; this is not an exhaustive or definitive account of that entire story, just some background, written so that anyone can understand it.
UPDATE: I have expanded the post to give readers a better idea of the contents of the link.