A U.S.-led military intervention in Syria would put Washington on a collision course with two unwavering allies of Syrian President Bashar al-Assad -- Iran and Russia.
Just how Tehran and Moscow might react is a key part of the calculus that U.S. President Barack Obama must consider in weighing his course of action in Syria.
Although analysts agree that neither country is likely to respond with direct military support for Assad, they also don't expect Tehran or Moscow to sit back passively. Iran's Islamic Revolutionary Guards Corps (IRGC) has said that an attack against Assad is a "red line" that would trigger a response, although it has not said what that response might be.
Iran's reaction to date has been mild, with Tehran condemning both the use of chemical weapons and threats of foreign military intervention.
According to Will Fulton, an Iran analyst with the American Enterprise Institute in Washington, the IRGC would likely not risk a direct confrontation with the United States but could act through proxies, including Hizballah in Lebanon or Shi'ite militias in Iraq.
Tehran would also seek to capitalize on anti-U.S. reaction at home and across the region. "I think we will absolutely see more condemnations, more warnings from IRGC and hard-line officials, and this will of course play into, especially, the IRGC's narrative that the conflict in Syria is a conspiracy of Israel and the West," Fulton says. "So they will use this attack to fuel that narrative and it will become a recruiting tool and a narrative defense of their own foreign interference in Syria."
Dina Esfandiary, a research associate at the International Institute of Strategic Studies in London, believes Tehran would prefer to limit its role in Syria, in part because Iran was itself a victim of chemical weapons during the 1980s war against Saddam Hussein's Iraq.
"If the U.S. and its allies are able to limit their air strikes and target only a few targets in Syria, I think that it's likely that Iran might react," Esfandiary says, "but they will really, really limit the extent of their reaction because as a country that has felt the impact of chemical weapons itself, it's very difficult for them to go against action against chemical-weapons use."
In any event, a strike on Syria would dim hopes that Iranian President Hassan Rohani's election last month could lead to improved relations
between Tehran and Washington. Rohani, who has pledged to improve relations with the West, could instead turn increasingly to longtime ally Russia.
Iran's Islamic Revolutionary Guards can be expected to use U.S. intervention as a recruiting tool, one analyst says.
In a telephone call initiated by Iran on August 28, Rohani discussed the Syrian crisis with his Russian counterpart, Vladimir Putin. The two presidents denounced the use of chemical weapons by "anyone," according to a Kremlin press statement, and called for a peaceful resolution of the crisis "exclusively through political and diplomatic means."
Russian President Putin is expected to have his first face-to-face meeting
with Rohani on the sidelines of a Shanghai Cooperation Organization summit in Kyrgyzstan in mid-September.
Bringing Tehran, Moscow Closer
Iran and Russia can be expected to work closely to protect their mutual ally, says Andrew Kuchins, director of the Russia and Eurasia Program at the Center for Strategic International Studies in Washington. "It is not clear to what extent the Russians and the Iranians are coordinating their support for Syria," he says. "But to the extent that they are, a strike on [Syria] would almost inevitably push Tehran and Moscow closer together to ensure that the Assad government does not fall."
Nikolas Gvosdev, professor of national security studies at the Naval War College in Washington, agrees. He argues that Moscow could ultimately respond to a strike against Syria by undoing years of Western policy to isolate Iran over its disputed nuclear program. "The longer-term prospect for the United States, maybe, is that Russia begins to weaken the international sanctions regime on Iran or begins to try to reintegrate Iran back into the family of nations in defiance of U.S. preferences that sanctions be tightened on Tehran," he says.
Gvosdev adds that Moscow could also respond to a strike against Syria by stepping up its direct provision of military aid to Damascus or by providing intelligence to mitigate the impact of an outside intervention.
On the other hand, Russia could see benefits in the United States becoming embroiled in another conflict in the Middle East. Such a scenario could allow Moscow to pursue its primary foreign-policy goal of reintegrating the former Soviet space through Putin's proposed Eurasian Union.
"If the United States becomes involved in a prolonged effort in Syria, beyond just a few military strikes at the beginning, but gets drawn into policing Syria, having to do more to prevent the use of chemical weapons, becoming involved in trying to stabilize the situation on the ground -- that means there will be a lot less attention paid to what goes on in the former Soviet space," Gvosdev explains.
U.S.-Russian relations -- already at low ebb in recent weeks, most notably over Moscow's granting of temporary asylum to former U.S. intelligence contractor Edward Snowden -- would certainly suffer as a result of a Western strike on Syria.
Russia could slow down or suspend the Northern Distribution Network that NATO uses to help supply forces in Afghanistan. Such a move could make things more difficult for NATO as its deadline for withdrawing combat forces from Afghanistan by the end of 2014 approaches.
"As we know, there are pits that you can fall into forever," says Aleksandr Golts, a military analyst and journalist based in Moscow. "Just recently it seemed that relations between Moscow and Washington had reached their nadir, but then came the Snowden affair and we understood that it is always possible to fall a little further."
RFE/RL's Russian Service correspondent Andrei Shary contributed to this report