Barack Obama resisted and agonized over the decision to intervene in Syria militarily for as long as he could. Last year, he even called off an imminent military attack to punish the Assad regime's alleged use of chemical weapons, after the Syrian government vowed to destroy its chemical arsenal.

But in the last couple of weeks, Obama reversed course in dramatic fashion. First, by launching a program to train and arm Syrian fighters, a move he had derided only a short time ago as trying to forge a military force out of "farmers or teachers or pharmacists". And second, by directly intervening in Syria militarily against the "Islamic State" through US air power.

Obama, a former constitutional law expert at the University of Chicago and Nobel peace price recipient, has tried to distinguish himself from his predecessor by conducting US foreign policy with a more multilateral bent and by trying to make US actions square broadly with international law - for instance in Libya.

No obvious legal justification

So, it is notable that Obama has so far refrained from making an explicit legal argument for his latest decision to authorize air attacks against IS in the Syria.

"The United States itself has not advanced a clear legal theory of its conduct," Nehal Bhuta, professor of public international law at the European University Institute in Florence, told DW. "Indeed Samantha Power the UN ambassador of the United States is on record as saying that they discussed options, but there is going to be plenty of time to talk about it. So I think they have expressly refused to articulate the legal foundation for these strikes."

What can be deducted however from what the US has said so far about a rationale for striking Syria is essentially that it considers it as a form of collective self defense.

According to Stephen Vladeck, a professor specializing in national security law at American University in Washington, the thinking is as follows: "That the Obama administration is basically protecting Syria, protecting the Assad regime, protecting the West, protecting Iraq from the threat posed by ISIS and that that's a valid exercise of self-defense under the UN charter and that it's a valid exercise of self defense under the authority of the US constitution under Article 2 to act in defense of the country even without an expressed authorization from Congress."

Controversial concept

There is a good reason the Obama administration has not talked much about the international law basis for the strikes against IS in Syria. That's because the concept of collective self defense is dangerous legal terrain.

"The notion of collective self defense is highly controversial and is still very much in its nascent stages", said Vladeck. "I think there is some support for it among international law scholars, but it's not as clear as if the US was responding to a direct attack on US soil."

The key question then is under what circumstances a non-state group may be attacked within a country if that country itself is not controlling that group. According to international law to justify an outside intervention, that country must be unwilling or unable to stop the threat itself.

While that concept is not easily applicable on the ground to begin with, the theory has been advanced by the US comparatively successfully in the case of attacks in parts of Pakistan, Yemen, Somalia and Iraq. But Syria is a different case.

Dubious legality

"Syria is a much trickier preposition," said Vladeck, "because on one hand part of why the Assad regime is in fact unwilling or unable to deal with ISIS is because we have been trying so hard to prevent them from dealing with threats like this."

Despite its controversial nature, for Bhuta the collective self defense argument is the best option for the Obama administration to bring its decision to strike Syria in accordance with international law. Still, he said, it is of "dubious legality."

"It's a difficult choice for Obama, but it's important to recognize that this is a choice that is in many aspects of his own and Western policy's making," added Bhuta.

His colleague Vladeck argues that the Obama administration is well aware of the tenuous legal rationale for its Syrian strikes and that this legal deficiency has a real effect on the ground in Syria.

"I think that's part of why what we are going to see at least in the short term is limited scatter shot strikes and not any escalation of hostilities at least not on the part of the US forces until or unless there is either some clearer legal predicate for larger scale hostilities in Syria or some material change in the facts on the ground."

Syria's role

And that could happen at any time.

"The real question is not whether international law scholars are convinced by this argument but whether the German government, the British government or the French government is convinced by this argument and at the end of the day whether the Syrian government is not going to object."

So far the Assad regime, despite some perfunctory statements rejecting the outside intervention, has remained largely quiet on the issue. Should Damascus change its tack and protest loudly against the attacks on its soil it could easily topple the shaky legal edifice erected by the Obama administration.

"I think the Obama administration is walking on eggshells, but at least thus far they haven't cracked any," said Vladeck.