Russia
Moscow has long-standing strategic and financial interests in Syria.
Syria hosts a Russian naval base on the Mediterranean, and contracts for Russian weapons sales to Syria — those signed and those under discussion — total $5 billion.
Moscow believes Western military intervention would not only infringe on Syria's sovereignty, but it would also create instability across the region.
"Russia's position is very easy to understand," says Andranik Migranyan, director of the New York-based Institute for Democracy and Cooperation, a nongovernmental organization funded by private Russian donors that is considered close to the leadership in Moscow.
"First, Russia is against any regime change from outside of Syria or any other country because according to Russia, any attempt to change the regimes, they are ended up in a chaos and results are quite opposite what were the intentions," he tells NPR's Robert Siegel. "This was proved in Iraq after the invasions of Americans over there. This was proved in Libya. This was proved in Egypt. And Russia is against principally this regime changes.
"And second, Russia would like to know, OK, Bashar al-Assad has to go. But who is coming next?" he says.
Daniel Treisman, a professor at UCLA, says the Kremlin's greatest fear is instability in the Middle East and Central Asia.
"Russian policymakers already worry about the northward spread of Islamic militancy and opium if the departure of NATO from Afghanistan leads to Taliban resurgence and state collapse," he wrote recently.
China
China and Syria have close trade links, but that isn't the only reason Beijing opposes Western intervention.
"The Chinese historically never supported external intervention and particularly the use of force against a regime," says Bonnie Glaser, senior adviser for Asia at the Center for Strategic and International Studies.
China might also have been burned by the intervention in Libya. Neither Beijing nor Moscow vetoed the U.N. resolution that eventually led to a NATO-led military intervention, which led to the ouster of Moammar Gadhafi. China believed that action went beyond the resolution's mandate.
China will continue to support Assad "until it becomes clear that the wind was blowing the other way," Glasser says.
"If the regime falls," she adds, "then the Chinese will quickly become silent.