So if the United States takes military action against Syria, what could be next?
The strike fails to deter dictator Bashar Assad from continuing his slaughter of civilians to put down a more-than-two-year rebellion. Such a flouting of President Obama could put pressure on the United States to ramp up military action, given that it has crossed the line into the use of U.S. military force.
2. Retaliation from Syria
Syria launches attacks on neighbor Israel to try to draw a response that would threaten to fragment the Middle East U.S allies that oppose Assad but do not want to be seen as supporting Israel - states such as Jordan, Saudi Arabia, Turkey and the United Arab Emirates.
3. Iran emerges
Iran has threatened often to use its naval vessels to cut off the Strait of Hormuz, a narrow point in the Persian Gulf off the coast of Iran where 20% of the world's oil passes. The move would force the U.S. Navy to respond with possible military attacks. Iran, a Shiite Muslim theocracy, can spur large Shiite populations in friendly Arab states to rise up against their leaders.
4. Terrorists strike
Terrorist groups in the Middle East, such as Hamas and Hezbollah, launch attacks against U.S. targets in the region and U.S. allies. Those targets can include bases such as the port for the U.S. Navy's 5th Fleet in Bahrain.
5. Russia gains
Russia, which under Vladimir Putin has increased repression at home and is steadily increasing its influence abroad, will become a major player in the Middle East by helping Syria survive the strike. That may strengthen the anti-American axis of Iran and Syria and lead to a further destabilization of a region of important strategic and economic value to Western nations.