Recently popular political movements have been anti-immigrant, anti-free-trade, and more generally anti-globalization. What these positions share is a lack of concern for outsiders. For example, U.S. discussions of the Trans Pacific Partnership (which has many advantages and disadvantages for everyone) tend to ignore completely the apparent large benefits for Vietnam. The technical term for this lack of concern is parochialism. In part, parochialism is part of our political language. The use of "we" in refers to fellow citizens, sometimes even excluding members of recently arriving ethnic groups. But some people, in their thinking if not in their speech, consider effects on outsiders, or even think of themselves as members of larger groups such as Europeans or citizens of the world. Once this kind of cosmopolitan thinking was even fashionable, as expressed, for example, in John Lennon's (1971) song "Imagine", and it seems to be coming back into fashion among some young people in Europe.
The simple argument against parochialism is that it is morally arbitrary, hence unjustified. The question of who should count in our moral judgments is a very basic one. The answer cannot be derived from competing philosophical approaches such as utilitarianism or deontology in general. So the usual attack on parochialism of any sort is to ask why a distinction should matter. This was the logical move made against slavery, racial discrimination, and discrimination against women. Of course, the defenders of these institutions sometimes tried to answer this attack by pointing to supposed empirical facts about, for example, how women's emotionality made them unsuitable as voters or office holders. But these arguments were ultimately recognized as post-hoc justifications, with little empirical basis. So the basic argument was, "If you care about what happens to X, why shouldn't you care equally about Y, even though Y is a different race, sex, or nationality?" This kind of logical argument is powerful, yet it is rarely made in public debates.
One counter-argument comes from a different analogy, loyalty to close kin. Equal treatment of everyone would imply that you should care about a stranger's child, spouse, or parent as much as you care about your own. If it is morally acceptable to give preference to loved ones, why not co-nationals too? This objection has several possible answers. One I like is that morality should concern itself with choices among options that are on the table. And the option to sacrificing one's own child for a greater good is not something that most of us would consider. We could just not bring ourselves to do it. (More precisely, our willingness to sacrifice our own concerns and desires is limited, so we should make our decisions so as to do the most good overall within this limit.)
Assuming that this argument works for loved ones -- and I think it does -- then could it also work for co-nationals? Yes, it could, if we feel such strong loyalty to our co-nationals. But we can take a step back and ask where our loyalty comes from. In the case of children, it is biologically determined. However, in the case of co-nationals, it is the result of an acquired abstract category. Even if humans evolved to be loyal to those in their immediate group of non-kin, the extension of group membership to total strangers requires a learned categorization of certain strangers as members of this group. Such categorization cannot plausibly be the result of natural selection, as it is, once again, arbitrary. If we can define "our group" as "German citizens", we could just as easily define it as "European citizens". People who reflect on this arbitrariness may come to change their loyalties.
In sum, it may be too late for those who feel very strongly about their co-nationals. From their perspective, parochialism can be justified, assuming that they cannot modify their feelings by reflection. Yet we can still object to the cultural forces that lead people to think this way, including the assumptions of political discourse itself.
A second line of argument for parochialism concerns the definition of responsibility that comes from the specific social roles. Social organization gives people decision-making authority in limited domains. When people violate these limits, they risk losing their authority, and they set a precedent for subverting a useful system. Police officers are not supposed to make decisions about punishment. That role is left for courts and judges, which are limited in yet other ways.
This is also a good argument, but is the role of a citizen just to advance what is best for their co-nationals. Many citizens do not limit their role in this way, and they are not even considered to be bad citizens as a result. Recent immigrants often think about others from their country of origin who might also want to immigrate. Some people take into account the effects of policy on other countries to which they have secondary loyalty. And still others think about issues that affect the whole world, such as climate change. We have no written rule against such a view of citizenship, nor any obvious social norms. The narrow definition of the citizen's role as serving only national interest is one that some people arrive at by themselves. It is not part of the social structure of roles, unlike the roles of police officers and judges.
Citizens do have a special responsibility toward their own nation, if only because they are in the best position to know what is good for it. They cannot rely on foreigners to decide on issues that have mostly local effects. But the exercise of this responsibility does not imply that outsiders should simply be neglected. It is a responsibility that applies much more to some issues than to others. As a citizen, we have a special responsibility to inform ourselves about national and local issues that don't have much effect on outsiders, and there are many of these. But just as our concern about city and state issues does not justify neglect of national issues, so our concern with national issues does not justify neglect the world outside.
In sum, the justification for parochialism of the sort we see in current politics seems weak. Would it be possible to confront people with arguments against this view in general? We don't know unless we try.