Perhaps there should be a follow up here regarding the reasoning of the decision.
(1) What is the reason people think it would be a good idea to allow hundreds of thousands of civilians be killed when it could be stopped with little cost to the already existent state apparatus?
(2) Is it fear of being dragged in some kind of prolonged conflict or some other pragmatic reason?
(3) Or is it some philosophical notion that "it's none of our business" ?
Ah, the fun questions have arrived!
(1) Three points: "good idea," "allow" and "little cost."
Is always intervening to prevent conflict a good idea? (Not always, so in x-amount of cases, it's a good idea not to intervene).
By 'allow', it seems that you're saying one is somehow responsible for remote conflicts, and/or that one has some obligation to intervene. Suppose there's some gang war occurring on the other side of your country. Was that your fault? Are you at all obligated to intervene in any way?
And regarding little cost, what do you mean? To each taxpayer, a war may be cheap in the short-run, but overall it's costly. A war/intervention can also be wasteful considering what else could've been produced/invested/consumed instead of the resources for intervention. An intervention can be especially wasteful if it becomes counter-productive because it could fail to resolve the systemic problems of the foreign conflict, or it could increase the genocide/conflict since the repressive government/rebels are being subsidized.
Finally, supporting a pattern of interventions maintains the "military-industrial-congressional complex" (MICC
), which arguably costs too much, doesn't keep the citizens secure efficiently, and causes much harm instead of good.
(2) Sometimes, but it's not just fear. It is also knowing one's constraints (and the constraints of a government). We can conceive of problems where one jumps into a situation without understanding what's going on and without having the requisite social networks/organizations which can alleviate that lack of understanding (knowledge problem). That kind of intervention can be well-intended yet problematic. Also the intervention occurs through an incentive structure which is not quite capable of actually resolving the conflict or even fixing the systemic problems (e.g. the US/NATO intervention in AFG, Iraq, Libya, and all the lovely examples of failed 'democracies' and sustained dictatorships through US/NATO intervention).
(3) So, (2) is the political economic approach. With philosophy, if one is libertarian, you'd be against coercing people to extract funds in order to pay a government-provided service to invade another country, which did not initiate conflict against you. Government-funded subsidies (e.g. foreign aid) are unacceptable as well. Of course, there's many other doctrines which can either reject or support intervention, but if they don't address the issues of #2, then those doctrines can become counter-productive (i.e. cause more harm than good).
So, in short, it is reasonable and a completely good idea to not intervene in some cases. Sometimes, we don't know what is best for others in particular situations, and even if we envision what is best (e.g. "world peace"), we don't know how to attain that goal of peace. In many cases, the well-intended means won't attain the goal. Sometimes, the well-intended voters are simply manipulated into supporting a particular foreign policy (e.g. fearmongering, guilt-tripping), and other times, the voters are completely removed from the foreign policy process after they cheered on the politicians but then change their minds (or simply lose interest).