As an immigrant, this is one of those things that still perplexes me about Americans despite my having grown up here. Nobody in Bangladesh thinks they should care as much about people in Africa as they do about their own neighbors. I see that attitude in the US, but it seems to mostly be an excuse not to really care about anybody except at the most diffuse level.
Additionally, if government officials are supposed to be governing the nation for the benefit of all mankind rather than looking after the interests of the people whom they theoretically represent, we might as well dispense with the pretense of representative government and try to formulate some other basis for state legitimacy.
It's not as if we don't have our own poor --blacks and whites.
Let's also remember, up and till WWi we were not a rich country. We were quite rural and not all that excellent. Many of today's poor countries were not that far behind the US.
Saying the civil war US was an example of a great army is like saying Syria has the greatest armies in the ME just because so many people are armed.
The US was rural, technologically backward, compared to some european powers, and not that far ahead of others.
Some of the issue comes down to fairness.
We set the rules. We have not achieved WH Bushes thousand points of light new world order single government. We look out for ourselves just as China, Mexico, Singapore do.
And as the most powerful, we cannot stand by and pretend everybody else is on their own. That is the most irresponsible thing any country can do. Even Poland helps their neighbors unconditionally in time of need. And they're no better off.