There has been little coordination between the two sides over the airstrikes. On the sidelines of meetings at the United Nations, Secretary of State John Kerry apparently told Russian counterpart Sergei Lavrov that the intervention was "not helpful."
Here are reasons both why that — as far as Washington's interests go — is probably true, and why it's not.
Russian President Vladimir Putin and other top officials have made no secret of their government's support for the embattled Syrian President Bashar al-Assad, whose regime has presided over the systematic destruction and unraveling of the Syrian state.
For the U.S. and its allies, Assad's departure has been a prerequisite to any lasting solution to the Syrian conflict, which has entered its fifth year and claimed more than 300,000 lives. But as Western drones and military jets streaked the skies over Iraq and Syria in the past year, little has been done to actually dislodge Assad, whose forces have also been battling the Islamic State.
Moscow has wrung its hands over the dangers posed by the Islamic State, an organization that Lavrov in April deemed "the main threat" to Russia's domestic security. Some 2,500 Russian citizens are believed to be among the jihadists' ranks.
But Russia has framed its war effort in Syria as an action taken specifically on behalf of the government in Damascus. As WorldViews noted earlier, the Russians have a long, close relationship with the Assad family that endured and, in some regards, grew closer after the end of the Cold War.
"Russia will factually be the only country to carry out this operation on the legitimate basis of the request of the legitimate government of Syria," said Putin spokesman Dmitri Peskov on Wednesday, gesturing to the supposed "illegality" of the U.S. air war in Syria.
A spokesman for the opposition Syrian National Council denounced the Russian airstrikes, telling The Washington Post's Hugh Naylor that Russia, like Assad, "targeted civilians, not ISIS."
It inflames sectarian tensions
The bigger worry is that, even if Russia does hit the Islamic State, its renewed support for the Assad regime makes an already grim, entrenched conflict even more intractable.
As the graphic above shows, Russia's recent deployments are centered roughly around Assad's remaining bastions in northwestern Syria, a coastal area that's the heartland of the Alawites, the minority sect to which the Assad family belongs. The contours of the Syrian conflict have shifted since the anti-Assad uprising in 2011, and now have a tragically sectarian character.
Sunni fundamentalist and Salafist groups have warred against Russians before, and experts believe they will benefit from Moscow's intervention. Saudi Arabia's Foreign Minister Adel al-Jubeir has labeled the new campaign a "dangerous escalation" and doubled down on his country's insistence on the removal of Assad, even with a "military option."
The kingdom, which sees itself at the vanguard of the Arab world's Sunni states, is believed to have directly or indirectly supported a number of Islamist and rebel factions fighting in Syria.
Those interests collide with those of Russia and Iran, a Shiite theocratic state and Saudi rival that has long propped up the Assad regime, recently with the direct backing of Hezbollah, Iran's Lebanese proxy. Russia's entrance into the conflict only deepens the region's larger geopolitical faultlines.
It makes the U.S. look bad
To some Republicans and neo-conservatives in Washington, the current scenario highlights a worrying contrast in style and policy between President Obama and Putin. To critics of the White House, Putin's apparent resolve and decisiveness in Syria is a consequence of supposed American fecklessness and dithering.
The Russian campaign "is a dramatic example of the diminution of . . . American influence in the region," warned Sen. John McCain, a perennial booster of the primacy of American military might abroad and the role it ought to play.
According to The Washington Post's editorial page, Obama has been unable to counter Putin's recent "flurry of alliance-building in Syria, Iraq and Iran" ahead of the airstrikes and could be compelled to compromise with a strengthened Assad regime. "Shortsighted and cynical as it may be, at least Mr. Putin has a plan for Syria," the editorial read.
It will make Russia look bad
For Putin’s entry into Syria, like almost everything else that he does, is part of his own bid to stay in power. During the first 10 years he was president, Putin’s claim to legitimacy went, in effect, like this: I may not be a democrat, but I give you stability, a rise in economic growth and pensions paid on time. In an era of falling oil prices and economic sanctions, not to mention vast public-sector corruption, that argument no longer works... And so his new argument goes, in effect, like this: “I may not be a democrat and the economy may be sinking, but Russia is regaining its place in the world — and besides, the alternative to authoritarianism is not democracy but chaos.”
Putin's boldness, in other words, is a fig leaf for a regime that's already in dangerous risk of over-reach. According to a recent poll, a minority of Russians back the intervention, while a considerable majority were "opposed or strongly opposed."
Given Moscow's limited military capabilities, at least compared to the U.S., there's little indication that Russia will be able to do much more to harm the Islamic State than the U.S. and its allies already have over the past year. The longer the campaign drags on, the larger the shadow of mission creep will loom.
If anything, Russian involvement in Syria will almost certainly divert its attention from Eastern Europe, which is good news for NATO and potentially Ukraine. It also risks alienating the Gulf states, Turkey and other regional actors, thus, if anything, blunting Russia’s effort to extend its influence in the Middle East. In the end, Russia’s Syria misadventure stands a good chance of doing Moscow much more harm than good.
It could damage the Islamic State
And what if Russia sticks by its word, and helps deliver some significant defeats of the Islamic State? Few organizations have commanded such universal global antipathy as the jihadist group, whose terror attacks, enslavement of women and beheading of hostages have filled international front pages.
This prospect likely explains the somewhat cagey response of top Western diplomats to Wednesday's news of the Russian airstrikes. Even if Russia focuses more on fighting other rebel factions, the potential gains for the Assad regime may be rather limited.
"The idea that Russian fighters will enable the [Assad] regime to reclaim territory is a fantasy," writes Hassan Hassan, co-author of a recent book on the rise of the Islamic State. "Moscow will bolster the regime’s capabilities to defend itself in key towns and cities, but nothing more."
A Russian government bogged down in Syria's complicated mire may better appreciate the need to phase the Assad regime out through some sort of political transition. At least that's the view of Obama administration officials who spoke to reporters of the New York Times. When asked about the Russian intervention, one official quipped: "Knock yourselves out."