If a muslim believes that other Muslims in their domestic country or outside of their borders are oppressed phsyically or mentally by, how should Muslims react to it?
Edit note: This question was merged here but was originally an answer to anoher more specific question.
Of course it is the duty of every Muslim to support the side of truth, especially when it is being oppressed. The question is then, how does one extend that support.
With a lot of people who want to "do something" - the question they should ask themselves is whether they got up for tahajjud that morning and cried in salah asking Allah to ease the pain of the Syrian people. I don't need to know the answer for each individual person, but if it is no then how can a person who cannot sacrifice their sleep go to sacrifice much more "doing something"?
The reason I say this is because like everything, there is a well-established fiqh for defending Muslim lands. Praying for Syria also counts as "doing something" in the eyes of Allah. In fact it is the first thing that a Muslim starts with. Then if you're capable of doing more, you try to learn more and figure out what is the best way to help, whether it's economically, politically, or whatever else. What is obligatory for us is to provide help in the department that they need the most, not the department that we feel like participating in - we might just be exacerbating the problem. And secondly, to provide help in a way that is legal according to the country you live in because you don't have to make things difficult for yourself or your immediate community.
Shaykh Haitham al-Haddad of the UK recently published a very nice piece after consultation with Syrian scholars on what kind of help they need the most: http://www.islam21c.com/politics/5616-do-we-go-for-jihad-in-syria . I highly recommend that for further reading and as a reference to my answer.