What was the role of the federal government in the acquisition of the west?
The federal government played a dominant role in acquiring the west of what is now the United States. The federal government acquired all of the land west of the Mississippi by treaty, war, or purchase.
The federal government started to acquire land west of the Mississippi with the Louisiana Purchase of 1803. When the government purchased the Louisiana Territory, it doubled the country’s land area. The new territory included all of the land in the Mississippi River basin up to the crest of the Rocky Mountains. All of this was bought from France for about $15 million.
Next, the federal government acquired the land that is now the Southwest of the US. Most of this land was acquired through war. The US took most of this land from Mexico after the war with that country in the 1840s. Technically, the US got Texas not through the war but by annexing the area, which had declared independence from Mexico about ten years earlier.
The federal government completed the acquisition of the Southwest in 1853. In that year, the government bought land that is known as the Gadsden Purchase from Mexico. This was land that is now in New Mexico and Arizona. It was bought for the purpose of building a transcontinental railroad.
Finally, the federal government acquired the Oregon Territory in 1846. This land had been jointly held by the US and Great Britain. In 1846, the federal government struck a deal with Britain to divide the territory, with the boundary being set at the 49th parallel. In these ways, the federal government was the entity that acquired all of the lands of the American West.
At this point in its history, the Federal government felt itself beset by enemies. It saw strong colonial intentions in North America by most of the European powers, plus it still feared war with Mexico. It is important to realize just how thin was its ability to project power into the West, and settlers there often sided with whoever provided protection from marauders and hostile Indian attacks. The Federal solution to this problem was to claim and develop all land to the Pacific Ocean so that no country could claim a legal foothold there. It helped that Europe was in political disarray -- Napoleon and the British royalty had weakened their own countries, new smaller colonial powers were emerging, and European countries were fighting among themselves and fighting for colonial territory in Africa, Asia, and the Americas.
As a result, and because the West was so inaccessible to European colonialism (compared, say, to Africa, the Middle East, and India), it was easy for the United States to impose its will on western territories and then annex them. It completed the Gadsden Purchase in the Southwest to secure a border with Mexico and the Oregon Territory in the Northwest to secure a border with Canada. These acquisitions gave the US access to ports, to lumber for ships, and to easy rail routes -- and nothing secures ownership more than developing access to the territories so settlers can come in. This same pattern was repeated in the north central US, with the annexation of the independent state of Texas, and other parcels.
It is worth noting that the Federal government learned immeasurably from the experience of Texas. The Federal government (basically a Northern or Union government at this point, shortly before the Civil War) saw Texas becoming a Confederate stronghold. It also saw the continued threats from Mexican and Indian forces in the settlements there, and the willingness of some Texans to ally with France. Texas was a unique case, but it shaped and drove the Federal acquisition policy through the rest of the century.