The answer has to depend on who "WE" are. If it is to mean the British Government (unilaterally) then in my opinion there is no way that can be justified simply on ethical grounds - interference in the affairs of another state on the basis of "knowing better" (wrong premise/assumption) or even being better (totally wrong premise/assumption) reminiscent of religious righteousness and posture of superiority.
However, if it is to mean carrying out a UN mandate then the question is rather different. While cynics will be quick to say that UN mandates are merely a rubber stamping of enforcement of the self interest of the most powerful, there is also the fact that military and other official violence within a country (self-on-self) should not be allowed to continue indefinitely to the logical end (complete domination by an elite) without something more serious than wringing of hands, tut-tutting and wailing outside that country's borders. That something has not yet been done to stop the excesses of the Mugabe regime in Zimbabwe is probably a major failure of the international community's moral fibre due in no small part to political correctness (no non-blacks to take action against a black ruler and fellow blacks are holding back for misplaced "brotherhood"). The ongoing absurdity of the Israel-Palestine conflict is another example of such failure, again due to a sort of political correctness (the well deserved sympathy for persecuted Jews who in Israel have now prostituted that sympathy to its complete expenditure/exhaustion).
When the UN authorises intervention then they need member states to volunteer the means to carry it out - the UN itself has no physical might. Whoever takes on the role of UN enforcer must then be scrupulous in keeping to the mandate and/or ensure it is clarified if difficult/impossible to uphold. The difficulty is that politicians and military leaders tend to get a bit carried away with their