R2P was adopted by the UN General Assembly in 2005 and by the Security Council in 2006. The first attempt by the UNSC to use it was against Burma military junta's crackdown in 2007, but this was vetoed by both China and Russia and went no where. There has been no subsequent formal attempt to use it until now.
http://en.wikipedia.org/wiki/Responsibility_to_protectThe Responsibility to Protect (RtoP or R2P) is a norm or set of principles based on the idea that sovereignty is not a privilege, but a responsibility. RtoP focuses on preventing and halting four crimes:
genocide, war crimes, crimes against humanity, and ethnic cleansing.<1> The responsibility to protect can be thought of as having three parts.
1. A State has a responsibility to protect its population from genocide, war crimes, crimes against humanity and ethnic cleansing (mass atrocities).
2. If the State is unable to protect its population on its own, the international community has a responsibility to assist the state by building its capacity. This can mean building early-warning capabilities, mediating conflicts between political parties, strengthening the security sector, mobilizing standby forces, and many other actions.
3. If a State is manifestly failing to protect its citizens from mass atrocities and peaceful measures are not working, the international community has the responsibility to intervene at first diplomatically, then more coercively, and as a last resort, with military force.
http://www.who.int/hiv/universalaccess2010/worldsummit.pdf"><2>
Threshold for military interventionsAccording to the International Commission for Intervention and State Sovereignty (ICISS) Report in 2001, any form of a military intervention initiated under the premise of responsibility to protect must fulfill the following six criteria in order to be justified as an extraordinary measure of intervention:
Just Cause
Right Intention
Final Resort
Legitimate Authority
Proportional Means
Reasonable Prospect
Criticisms
RtoP and National Sovereignty: One of the main concerns surround RtoP is that it infringes upon national sovereignty.
RtoP Scope too Narrow: The scope of RtoP is often questioned. The concern is whether RtoP should apply to more than the four crimes: genocide, war crimes, crimes against humanity, and ethnic cleansing.
Use of Military Intervention: The question of military intervention under the third pillar of RtoP remains controversial. Several states have argued that RtoP should not allow the international community to intervene militarily on States, because to do so is an infringement upon sovereignty. Others argue that this a necessary facet of RtoP, and is necessary as a last resort to stop mass atrocities. A related argument surrounds the question as to whether more specific criteria should be developed to determine when the Security Council should authorize military intervention.
There are other pros and cons at the R2P's wiki site.