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As the S-300s were arriving, a Syrian delegation was in Moscow attempting to buy more arms. A Russian official said Moscow would provide 10 new MiG jet fighters designed primarily for air defense, not ground attacks on rebels.

Sen. John McCain, the Arizona Republican who recently met with Syrian rebels, said Tuesday that Russian arms shipments show that it is fruitless for Mr. Obama to hope that Mr. Putin will nudge Mr. Assad aside.

“When it comes to the administration’s policy toward Syria, to say they are ‘leading from behind’ is too generous. That suggests they are leading. They are just behind,” Mr. McCain said. “In its desperation, the administration now appears to be placing its hopes in the Russian government to push Assad from power. … This is the same Russian government that continues to provide heavy weapons and moral support to Assad, that refuses to authorize U.N. sanctions on the regime, and that even blamed Assad’s recent slaughter of civilians in Houla on the opposition and foreign powers.”

Robert Springborg, a professor at the Naval Postgraduate School in Monterey, Calif, said Mr. Putin is making a big mistake, long-term.

“Why not call it a Putin-Assad alliance — an alliance of two dictators clinging together in the face of domestic, regional and global challenges?” Mr. Springborg said. “The alliance is not in the strategic interest of either country, eroding whatever support there is for Russia globally and regionally, while further alienating the [ruling] Alawites of Syria from the Arab world within which they must live, to say nothing of cutting Syria off from the [Persian] Gulf and other sources of investment.

“This is the story of two drowning men clutching on to one another. We have every interest to ensure both drown,” he added.