The belief that the u. s. was a "melting pot," which became very compelling in the first part of the twentieth century, suggested that the nation had an almost divine mission to produce

A melting pot, is a pot into which many different substances are going in, and then as they melt, they become fused with each other, - producing one substance. A melting pot as a metaphor means that the US was to produce a homogeneous society, a mix of all the constituent societies, which, it is to be understood, would be stronger than all the constituent societies by themselves.