DW: How would you describe and rate Obama's Syria policy over the past four years?

Robert Ford: The Obama administration has been unable to contain the Syrian crisis as it had hoped. Syrian refugees were the majority of those who flooded into Europe in 2015, causing new political tensions inside the EU, a vital American partner. Syrian extremists helped organize and execute terror attacks in France, Belgium and Germany, all US allies; the "Islamic State" helped inspire American extremists in California who killed a dozen people. The US for nearly two years has been bombing inside Syria and there is no end in sight, despite progress on the ground. The administration's big claim of success, the destruction of Syrian government chemical weapons capabilities, is limited; the Syrian government continues to use chlorine gas with impunity.

The most likely prospect is continued fighting, and more refugees, until the Syrian government on one side and extremist elements within the larger Syrian opposition all agree to a ceasefire. That ceasefire appears far away and will only result in a de facto unstable partition of Syria.

In its last five months in office, the Obama administration is unlikely to shift its tactics or its strategy very much. The new US administration will confront an "Islamic State" (IS) which is slowly losing territory but is also preparing to return to the insurgency from which it sprang in 2014. Meanwhile, the former al-Qaeda affiliate in Syria is stronger than ever, and Russian bombs and Iranian-backed militias in Syria, along with what is left of the Syrian army, cannot defeat it and the rest of the Syrian opposition.

Gordon Adams: The Obama policy has not reversed the trend in Syria. But it could not change much. Basically, the US has very little leverage over the players - the Turks, the Iranians, the Saudis, Bashar al-Assad, the Russians, Hezbollah, among others. What the US lacks in Syria is leverage. So almost regardless of what Obama had done, the crisis would have progressed the way it did.

The carnage in Syria continues

Andrew Bowen: President Obama's initial assessment that Syria could be kept in a box proved to be a miscalculation. It's not so much this miscalculation, but his overall aversion to being drawn into new wars in the Middle East which has driven Obama not to take any pro-active action to resolve Syria's civil war.

Obama reluctantly was about to take action on his "red line" remark and was very content to be able to pull back from the rhetorical trap he placed himself in. The president has treated Syria as a challenge that could be delegated to largely lackluster diplomacy. The refugee challenge was largely left to Europe and Syria's neighbors. Obama watched with passive disinterest as Russia moved last fall to militarily shore up President Assad's regime while Iran continued to entrench itself in Syria.

Watch video 01:03 Obama turns up pressure on Putin over Syria

What alternative path could Obama have taken?

Robert Ford: There are two alternative paths that could have been taken. One might, perhaps, have changed the war. The other would not have changed it much.

A. The Obama administration could have stayed entirely silent, not urged Assad to step down in August 2011, and remained an observer. This would not have changed Turkish/Saudi/Qatari behavior nor would it have changed Russian/Iranian behavior. The war, the destruction, the refugee flows and the rise of extremist elements all would have occurred even with the US entirely out of the conflict.

B. The Obama administration could have decided in autumn 2012 or early 2013 to strongly back moderate elements in the armed opposition in return for those armed opposition groups avoiding sectarian behavior and reaching out politically to elements of Syrian society still backing Assad. This would have meant much larger material and cash assistance so that the moderate groups could have successfully competed with the al-Qaeda affiliate Nusra Front (remember, the Islamic State didn't exist in 2012 or early 2013) for recruits.

The failure to step in strongly in 2012 or early 2013 gave al Qaeda and other relatively well-funded hard-line groups a big advantage in recruitment. The Obama administration's failure to enforce the "red line" in a meaningful way that stopped ALL Syrian government chemical weapons attacks further fueled extremist recruitment. The State Department specifically warned of that risk. The White House ignored that counsel.

Gordon Adams: There were no good alternatives. A no-fly zone would have enmeshed the US in the war without providing a prospect of success. The "moderate" opposition scarcely existed, so more arms and training would not have accomplished much. An airstrike over the chemical weapons would not have taken down Assad, not without an invasion, which would have been counter-productive. Only cooperation with the Russians, who have real leverage in Syria, might have accomplished something, but even that is doubtful.

Andrew Bowen: Obama had a number of opportunities to more robustly train and arm moderate opposition fighters. The president could have taken robust military action to cripple President Assad's Air Force in the wake of his "red line" remark. He was presented numerous times with "no-fly zone" options, both in the north and the south, which he didn't take.

Obama never wanted to match active diplomacy with credible force if diplomacy failed. As a result, both Moscow and Tehran have never taken Obama's words and intentions seriously and don't see much consequence if these talks fail. As a number of senior regional officials have noted, it's now a race to see how much that can be gained before a new administration enters office in Washington who may push back. Even if a President Clinton wants to take more action, Obama's leaving her a poor hand to play.

Russia has propped up its Syrian ally

Obama's "hands-off" policy leaves the US four years later with a stalemate that has not only produced one of the world's worst humanitarian crises and a vacuum space for IS to thrive. It's also enabled Russia and Iran to set the tone of the conflict and the terms of the settlement.

Robert Ford was the US ambassador to Syria (2010-2014) and to Algeria (2006-2008). He is currently a senior fellow at the Middle East Institute.

Gordon Adams is professor emeritus of US foreign policy at American University and a distinguished fellow at the Stimson Center.

Andrew Bowen is a global fellow in the Middle East Program of the Wilson Center.

The interviews were conducted per email by Michael Knigge.