If I put on my "actual beliefs" had, the reason I support foreign aid is that it does very large amounts of good, and I am a utilitarian.

If I try to take in the assumptions of common-sense morality, then I think there's a similar rationale. Promoting the impartial good isn't the only purpose of government. Maybe governments have way more obligation to their citizens than others. But still, it seems like one important purpose. It would have been noble to stop the Rwandan genocide at minimal cost. One feature strongly counting in favor of ww2 intervention was that it stopped the Nazi holocaust.

It seems plausible that the tiny slice of money spent on foreign aid does more good than the rest of the federal budget, barring a few necessities like preventing America from being invaded. For this reason, I think commonsense supports foreign aid.