Dapo Akande has a pithy analysis of the legality of humanitarian intervention at EJIL: Talk! Akande notes that “there is very little State support for the view that international law permits States to use force in other States on humanitarian grounds.” He adds:
The UK is of course one of the few States that does accept that international law provides a right of humanitarian intervention. However, this view has been rejected by the vast majority of States. See for example the 2000 Declaration of the South Summit by the G77 composed of about 130 member States ['We reject the so-called “right” of humanitarian intervention, which has no legal basis in the United Nations Charter or in the general principles of international law'. para. 54 ]. Also, even other European States have failed to advocate such a right. In the ICJ proceedings regarding the Legality of the Use of Force (by NATO in Yugoslavia), only the UK and Belgium expressly relied on the doctrine of humanitarian intervention. Other NATO countries refrained from doing so.
In short, there is little opinio juris on which a doctrine of customary international law might be based. Although the matter has not been expressly considered by the ICJ, the Court did seem to reject the doctrine of humanitarian intervention in the Nicaragua case (1986). In the case, the Court stated that: “while the United States might form its own appraisal of the situation as to respect for human rights in Nicaragua, the use of force could not be the appropriate method to monitor or ensure such respect.” (para. 268). The argument that international law today allows for unilateral right of humanitarian intervention is very weak. No such right exists in international law as it currently stands.
He then addresses the “responsibility to protect” doctrine:
The 2005 World Summit Outcome document, agreed by consensus at Head of State level in the UN General Assembly, is the definitive document on what the “R2P” doctrine is intended to mean. Para. 139 of the document speaks of collective action, through the Security Council, should peaceful means fail. In other words, forceful action to prevent mass atrocity crimes is reserved to the Security Council. The notion that where the Security Council is deadlocked, “R2P” provides a legal framework for the international community to use military force – either by way of a regional coalition or a so-called “coalition of the willing” is just fanciful.
Akande also analyzes the theoretical hurdles to recognizing a customary international law rule of humanitarian intervention that overrides the U.N. Charter.