Manifest Destiny was a distinctly racist concept in the first place. The idea was that white man was destined to conquer the west and this meant subduing anyone else who happened to live there. White man alone was entitled to this "destiny". It used the ideas of Jacksonian democracy of a "white man's democracy" to develop these beliefs.
Manifest Destiny is a term for the attitude prevalent during the 19th century period of American expansion that the United States not only could, but was destined to, stretch from coast to coast. This attitude helped fuel western settlement, Native American removal and war with Mexico.
Droughts and severe winters were the main problems facing western cattle barons