What has made the United States "great"

The greatness of the US isn't due to what it has to work with, but instead what it represents. It is "the land of dreams". Where success can be made.
I wish to give some points about immigrants here and immigration in general. The people who come into the US aren't your average people. The strict rule follower, those happy with their lot. It is the dreamers, those with ideas and a striving need to reach at them. The States is where this dice roll is most likely to be pushed in a positive way.
All citizen fucking love their countries, everywhere. They love their families, communities and their way of life. Most have the addition of being 'ok' with their lot/place in which they fit. Within that mindset your neighbors often are your enemies, the States is a movie, or TV show, with their xenophobia and prejudices added.
More than likely the US has injured their country and people, even family.
The US doesn't exist anymore than "a galaxy far, far away" in the usual isolationist attitude most carry around.
Now the rare few. Those thinkers, provers, again dreamers willing to leave the country they love, their family and way of life, even very often their language, hat in hand, move towards the States as a possible answer to their forward thought. These Humans come in with some amount of financial stability, higher education, all these boxes need to be triple checked to come in above the board. These, again legal processes are extremely stringent, very drawn out and expensive.
Those in the "Caravan of Doom" are in a slightly different category. These are refugees. These are people whose lives and the lives of their families are under threat. Immediate threat.
The "war" they are fleeing is a war we've, the US citizens, made. That we funded and fund. That we implemented and maintain.
So terrible are their lives of these Humans, they are willing to leave all behind, a bag is all most have, escape the country they love. Then under threat, they cross the Mexican Southern Border. The way far too many people feel about basically everyone living to the south of the US/Mexico Border is the way the Mexican people feel about those to their south.
Cross through profound animosity, distance and those who pray on those in need, just to get to the United States border. Simply to get somewhere where they have a chance to survive. Not to seal jobs or virgins, to fucking breathe a few more times.
Those south of "our" borders aren't little, insignificant people. Our policy of "so long as they aren't commies alls fine" is what wrought the situation they are fleeing.
Those coming north aren't to be feared nor loathed, only pitied in a compassionate way for their struggle. They need help and need to be helped and we have the ability to help them and others. Had we helped earlier in the stabilization of these and basically all countries, they wouldn't be coming north.
These are scared, threatened people, only seeking safety.
They aren't coming for our jobs, to be on welfare, to take. They are only wanting safety. Once safe they can build their lives. Work in the shit jobs we would never do unless post prison. Pay taxes. Rent. Pay towards education for English if needed, then a trade they can do. These are not slum dwellers, murders, rapists, uneducated, these are the middle class made poor by their countries situation. People simply seeking refuge, refugees of our wars happening just to the south. East. West. Sorry one problem at a time.
Try and be a little more Human and a little less a chimp.

Good luck out there