To what extent do states intervene in some humanitarian crises and not others?

the structure will be like:

first introduction defining humanitarian intervention and which states are likely to intervene specially USA AND RUSSIA

main paragraph will be in two parts
first : Syria and how these state intervene also theories should be included ( realism and liberalism )
second : Libya > the same and the theories important

i will add some information as well next week to help write the essay very clear and as it requested to me.