What did author Ralph Waldo Emerson mean when he said "Mexico will poison us"? Was he right? Why or why not?

Speaking about the Mexican-American War, Emerson states that America will conquer Mexico, but it will be like swallowing arsenic; it will poison and kill America itself. America had annexed Texas and then gone to war with Mexico over it, eventually winning and taking the land as its own.
This war and conquest took place during a period of massive expansionism in America. Manifest Destiny was the way of the land, and the country was stretching itself farther and farther west. Emerson was saying that, by defeating Mexico, America had convinced itself that it truly did deserve all the land it was taking, and it was tainting American principles. The idea of Manifest Destiny, and slavery along with it—as slaves were being used to build these settlements—was seeping further into the American psyche and its politics. These moral compromises and the entitlement the government was beginning to feel would poison America and eventually bring it to ruin, and, according to Emerson, it would stem from the defeat of Mexico.
In a sense, his words were prophetic, as these issues soon led to the Civil War.