Russia has muscled the US out of Syria

Russia and Syrian forces appeared to have let up on bombing besieged Aleppo on Tuesday, possibly allowing civilians and rebel groups to flee the city.

While citizens and combatants in Aleppo would likely welcome any cessation of the brutal Russian and Syrian air campaign that has been linked to possible war crimes like bombing hospitals and using banned munitions, the unilateral move completely excludes the international community and the US.

To put it bluntly, the US and larger international community have been muscled out of Syria by Russia.

The US no longer "has a horse to ride" in Syria, according to Cliff Kupchan, chairman of Eurasia Group and an expert on Syria and the Middle East.

Kupchan says that two key events have undermined the US's clout in Syria to the point that there are no longer any good options there for the US to pursue.

The rise of Jabhat Fateh al-Sham - an Islamist group formerly affiliated with al-Qaeda that called itself Jabhat al Nusra - as the dominant opposition group to Syrian President Bashar al-Assad's regime represents the first blow to the US's strategy of "training and equipping" secular, moderate rebels.

Kupchan says that towards the beginning of the Syrian conflict in 2011, there was secular opposition to Assad that had "sufficient numbers to balance" Assad's forces on the battlefield.

Now, however, "the secular opposition is extremely small and not terribly relevant to the ongoing Syrian tragedy," Kupchan said.

After the erosion of meaningful opposition to Assad, Russia's intervention on behalf of Assad just over one year ago may have been the final nail in the coffin for the US and larger international community's relevance.

"Assad was on his heels," said Kupchan. "Russian intervention put him back moving forward. Assad and his allies are now the dominant power on the battlefield."

In fact, Russia dominates Syria's airspace so much that the US can't really consider launching airstrikes against Assad - Moscow's recent deployment of advanced surface-to-air missile defenses to Syria have created a de facto no-fly zone over areas under government control.

Dr. Igor Sutyagin, an expert on Russian missile defenses at the Royal United Services Institute, told Business Insider that the US would now strain to strike at Syrian government targets. Even the US Air Force's fifth-generation stealth aircraft would have to be "operationally, tactically brilliant" to strike Russian or Syrian targets.

But as Kupchan points out, even if the US could readily destroy the missiles, risking war with Russia is not worth it.

Since the kinetic solutions to ending the war have crumbled, US officials have been again discussing the idea of equipping and training rebels and the Kurds, but Kupchan points out that these are not good options either.

"In theory, there are sufficient secular and non-extremist members of the opposition that one could construe a train and equip program. The problems facing that strategy would be numerous - the Syrian/Russian/Iranian alliance has significant momentum on the battlefield, and while we may well give weapons to the 'sane opposition,' the chances they would end up in the hands of the radical opposition is extremely high," said Kupchan.

Indeed, US efforts to train and equip groups in Syria have failed spectacularly in the past. According to Kupchan, "the best shot is the shot we're taking - to continue to engage Russia and, to the extent practical, the broader community internationally (Turkey, Saudi Arabia, and Iran) to try to save lives."

But when Syria, Iran, and Russia increasingly control the situation on the ground, the prospect of leveraging these countries to act against their interests, with no threat of military challenge from the US, grows increasingly dim.

The US and international community can continue to nudge and prod Russia to do the right thing in Syria, but Washington's chances of extering real influence on the ground have now dwindled to virtually zero.