Image caption The Kosovo intervention in 1999 was justified by humanitarian concerns

The words "international law" convey the sense of a set of established international rules and authorities agreed by all nations, and easily understood and applied by them.

Sadly that is far from the case, and in practice, it is difficult, if not impossible, to get definitive rulings in international law involving military intervention. There is no international court on hand to give the legal go-ahead to intervene.

However, there is a developing legal framework for military intervention on humanitarian grounds.

Known as the Responsibility to Protect, or R2P, it was born out of the humanitarian disasters of the 1990s in Kosovo and Rwanda.

It is widely but not universally accepted and has three principal elements:

States must protect their own populations from genocide, war crimes, ethnic cleansing and crimes against humanity, while, simultaneously, the international community has an obligation to help states prevent such crimes

Where there is strong evidence of these crimes and a state cannot or will not stop them, the international community should exhaust all peaceful means in seeking to bring the atrocity to an end

If all that is done, and fails, the international community can use military force

In order to have maximum legitimacy, military intervention should be authorised by the UN Security Council. It holds a unique position as the primary arbiter on the use of force in international law.

However, as in the case of Syria, it may be hamstrung by a lack of consensus, with one or more members opposed to action.

'Coalition of the willing'

In these situations, according to one view, R2P provides a legal framework for the international community to use military force as a last resort - either by way of a regional coalition or a so-called "coalition of the willing".

There are a number of safeguards in R2P:

There needs to be powerful evidence of an ongoing atrocity

Peaceful measures, such as diplomacy and sanctions, must have been exhausted

Any force used must be specifically targeted at stopping the atrocity and protecting the civilian population

In other words, it is a limited power to act. However, if all of the criteria are met, then the limited and targeted use of military force would be legal in international law under R2P, some lawyers and commentators believe.

Ultimately though, military interventions in these circumstances are up to governments rather than lawyers.

It is for them to make the case for military intervention by showing that the legal requirements have been met.

In the case of Syria, they will argue that there is an ongoing atrocity, all peaceful means of stopping it have been exhausted, and that targeted military action could achieve the twin goals of ending the atrocity and protecting the civilian population.