What was the role of the federal government in the acquisition of the 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.