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:
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.
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:
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.
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.