What is our global responsibility?

This is a complex geo-political question. Many people have criticized our invasion in Iraq (including myself) but can not deny that Saddam was a miserable dictator. However, at the same time, the same group of people has urged the US to enter Darfur, Sudan and other war-stricken countries to help tame the violence. What are the responsibilities and obligations of the US (and other global superpowers) in terms of conflicts and inhumanities in foreign nations? Are we justified to enter other nations, no matter how cruel and inhumane it is??