I hope this info can help you, let me explain some important parts of what you need: First it was the Buffalo soldiers, not cowboys. They were blacks formed into regiments in the US army, under white officers. They had at least one cavalry regiment and infantry regiment, and fought well and bravely on the frontier. The second thing you need to know is that the belief is based on the fact that the United States was intended to expand. Its growth was supposed to be across the continent and it was used by democrats in the 1840s. It was used as a justification of the war with Mexico