Why should the United States care about Syria?
A lot of people are dying in a terrible conflict that shows no signs of abating. Syria is surrounded by U.S. allies - Israel, Jordan and Turkey - which are all affected by the war. It threatens to spread instability, sectarian conflict and political competition in a part of the world that has plenty of all three. Syria is a crucial ally for Iran, which means the war has big implications for Iran's foreign policy and its tenuous relationship with much of the world. The war is a growing haven for Islamist extremists, including groups allied with al-Qaida.
The question of what the United States should do about Syria, if anything, is also a proxy for larger questions about America's role in the world and its willingness to use force. It touches on the legacy of the Iraq war, but also on how the United States deals with international institutions and humanitarian crises abroad.
Syria is a catastrophe. What does the United States stand to gain by getting involved?
The Obama administration's primary case is that the United States has a responsibility, as a global leader, to punish Syrian President Bashar Assad for using chemical weapons. The world has a fragile and hard-earned understanding that chemical weapons are not used in wars, no matter how brutal. If the United States (or any other nation) doesn't rebuke Assad, the global stand against chemical weapons will be weakened, opening the door to possible use of chemical weapons in the future. Preventing that would be good for the United States, because we don't want to live in a world where people use chemical weapons and because we want to continue to be perceived as a global leader.
The administration also argues that the United States needs to protect its "credibility." Obama had earlier called chemical weapons a "red line"; if he doesn't respond to that line being crossed, some worry that foreign leaders might take U.S. threats less seriously in the future. The administration also says the United States has immediate national security interests in mitigating Syria's war, which threatens to spread into neighboring countries, although the strikes would be less about containment and more about deterring the use of chemical weapons.
What does the United States have to lose by launching strikes?
It's possible that strikes won't actually deter Assad from using chemical weapons and could even make him more likely to use them if he panics. Strikes could also inflame anti-Americanism in Syria or the region. And cruise missiles could kill innocent civilians.
The biggest concern, though, is that the United States could get sucked into a war it has worked hard to avoid. Mission creep happened after the United States invaded Iraq and Afghanistan; it also happened in Libya in 2011, when a "no-fly zone" grew into an all-out intervention against Moammar Gadhafi. The danger that limited strikes will become something more open-ended is real.
What if Syria or its allies strike back?
The Syrian military does not have much capability to strike back. More to the point, it's not as interested. Assad doesn't want to escalate a conflict against the most powerful military on the planet.