Us vs Them happens when there is competition. Competition happens when there is demand for limited valuable resources and not all that demand can be satisfied.
We bring the people of the world and the people within our nations closer together by eliminating absolute poverty.
Eliminating inequality is neither necessary nor even desirable, but the floor of our economies must be such that those sitting upon it enjoy a certain minimum acceptable level of comfort.
Behavioral economic research shows this isn't how humans think, particularly Americans. They are naturally competitive regardless of their base level of comfort and "fairness" is inherently very important to them. If one would get 1 unit of free utility and another person gets 5 units of free utility, but the person who gets 1 unit has unilateral control over whether or not anyone gets anything, a startlingly high percentage of people in the 1 unit group will completely axe the deal entirely despite being better off.