Despite a decision by the British Parliament not to authorize military force in Syria, the United States has said it will decide on its own how it will proceed in confronting the Syrian regime.

Backed by the French, the Americans justify their intervention on the grounds of the poison gas attack on several villages near Damascus last week for which they hold the President Bashar al-Assad's regime responsible.

Obama on Friday said he was looking at a "wide range of options" for military intervention in Syria but also stressed that none of them would involve putting "boots on the ground."

US Secretary of State John Kerry called the alleged use of chemical weapons a violation of international law that would legitimize a US military intervention. Because the UN Security Council was unable to decide on a military intervention due to Russian and Chinese opposition, the United States decided to forgo a resolution and attack Syria without the council's approval. Such a unilateral intervention is controversial from an international law perspective.

Many states oppose intervention

Humanitarian intervention remains highly controversial in international law, according to Stefan Talmon, who teaches international law at the University of Bonn. Most nations, he said, reject humanitarian intervention.

"From a Western perspective, humanitarian intervention may be permitted today only in extreme cases when an imminent humanitarian disaster can be averted through such intervention," he said. "But from a global, humanitarian intervention is not acceptable. And under international law, of course, the global perspective is decisive."

The concerns of intervention opponents are based on previous experiences with unilateral interventions, particularly the US invasion of Iraq in 2003. The US justified that war by claiming Saddam Hussein's regime possessed weapons of mass destruction. The claim proved, however, to be not only unfounded but also based on knowingly false information to justify the intervention.

Interventions that are not sanctioned by the UN thus stand under suspicion of being arbitrarily conducted or for reasons other than those declared.

Danger through misuse

"Of course, humanitarian intervention poses a risk, especially for weak nations that are incapable of humanitarian intervention themselves but can become the victim of such intervention," said Talmon, pointing to the attack on Iraq as an example.

Humanitarian intervention poses significant potential for abuse, "especially when it is carried out following the decision of a few nations - a so-called coalition of the willing," said Talmon.

Following the Kosovo conflict, the Group of Non-Aligned States, consisting of nearly 120 countries around the globe, expressly established that "the so-called right of humanitarian intervention has no basis in international law," said Talmon.

"Even now we will hear states, not only Russia and China but also probably Brazil, India and South Africa, speak critically about military intervention if it doesn't take place on the basis of a United Nations resolution," Talmon added.

Responsibility to protect

This difficultly is also not resolved through the UN's recognized principle of the responsibility to protect, according to Talmon. Under that principle, states commit to take action in the most severe violations of human rights. The action should take place, if possible, in the form of preventative measures but, in the event of an emergency, it could also be in the form repressive measures, using violence. But, here too, there is no legal basis for individual states to intervene militarily.

In its repressive variant, the concept of the responsibility to protect implies only that the Security Council, which was originally intended to become involved in international conflicts, can and should now intervene in internal conflicts as well – but only within the framework of the general limitations of the UN Charter. That means that members of the Security Council can agree to an intervention. But this also doesn't justify an intervention in Syria, according to Talmon, because there is no agreement about Syria in the UN Security Council.