The Responsibility To Protect is a marketing idea. The primary duty of a government is defend its people against enemies whether internal or external. Some do it better than others. Some are worse. Others are dreadful. The UN is claiming the right to invade in extreme cases.
Casus Belli
Is the case for war, in Latin of course. Justification is another matter.
http://en.wikipedia.org/wiki/Declaration_of_war
http://en.wikipedia.org/wiki/Laws_of_war
Responsibility T o Protect
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The 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, which it places under the generic umbrella term of, "Mass Atrocity Crimes". The responsibility to protect can be thought of as having three parts.
A State has a responsibility to protect its population from genocide, war crimes, crimes against humanity and ethnic cleansing (mass atrocities).
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.
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.
In the international community RtoP is a norm, not a law. RtoP provides a framework for using tools that already exist (like mediation, early warning mechanisms, economic sanctioning, and chapter VII powers) to prevent mass atrocities. Civil society organizations, States, regional organizations, and international institutions all have a role to play in the operationalization of RtoP. The authority to employ the last resort and intervene militarily rests solely with United Nations Security Council and the General Assembly.
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This is an excuse for invading small countries that cannot hit back. Why not just steal their oil and have done?