The Louisiana Purchase helped to transform America’s view of itself by making it seem more as if the United States was destined to be a large and important country.
Before the purchase was made, the US was not very big and was rather weak. The purchase did not strengthen the US militarily, but it did increase the size of the country tremendously. All of a sudden, the idea of a country that stretched across the entire continent was not unreasonable. Americans came to feel that the country could be physical very large, and presumably powerful in proportion, in the future.
The Louisiana Purchase, then, caused Americans to see their country as a potential great power and helped to give them a greater sense of national pride.
The Louisiana Purchase did much more than double the size of the United States. Before getting into the more long-term effects of the purchase, it is important to note that when the United States purchased the Louisiana territory for 15 million dollars, the government was not purchasing the huge tract of land that they ended up getting. The purchase was initially for the city of New Orleans and the land around it for 10 million dollars. It was not until Robert Livingston was sitting at the meeting that the French Treasury Minister, Francois de Barbe-Marbois, offered the remaining huge portion of land for only 5 million dollars more.
The first point of deepened understanding due to the Louisiana Purchase was in regard to the US Constitution. Ambassadors Robert Livingston and James Monroe set up the purchase. Livingston and Monroe said yes without consulting President Jefferson or Congress. Jefferson went along with it, but he doubted that the Executive Branch had the authority to expand the United States by acquiring new land. The purchase was set up through the treaty that the Senate approved.
The House of Representatives was not as happy about the purchase. Majority Leader Randolph tried to undermine the House of Representative vote and went further to claim that the territory purchased actually belonged to Spain. Most of the worry was over the New England states losing power due to an expansion westward. There was also concern that this new territory would cause problems with:slavery, representation, and a possible overthrow of the United States government by the newly acquired French citizens.
Spain did argue that the land was theirs. Madison handled the complaint by successfully arguing that when the United States approached Spain to buy the territory a few years earlier, Spain had declined and told them that it was French property. (Reference #1) The Louisiana Purchase was a major influence in the United State's foreign position in the world. While it connected the United States with France through treaty, it also solidified the US position with Great Britain through trade.
The constitutionality debate of the Louisiana Purchase was not resolved until 1920 with a Supreme Court Case called Missouri v. Holland. The decision, reinforced the "supremacy clause" (Article V1, Clause 2). It recognized state concerns over parts of treaties but made treaties the supreme law of the land.
Domestically, the Louisiana Purchase affected the United States in three distinct ways: a) Slavery. The House of Representatives was correct in their assumption that the purchase of such a large territory would bring about issues regarding the slavery issue. Several controversial pieces of legislation (Missouri Compromise, the Compromise of 1850, and the Kansas-Nebraska Act) were all implemented in order to resolve these issues. The controversy regarding slavery in the territories will culminate with the United States Civil War in 1860.
b) The second domestic issue was also predicted by the House of Representatives: a significant change in the US Population. The Louisiana Purchase brought in several cultural groups that were largely unknown prior to Jefferson's presidency in the United States including Catholics and the French. To a largely protestant Anglo-Saxon population this was a major shift in the way the United States looked at itself in regard to immigration. Many historians have noted it as the first moment of the "melting pot" in US History.
c) Finally, the Louisiana Purchase is largely responsible for the decline of the Federalist Party in the United States. The new states created due to the Louisiana Purchase expansion helped Jackson rise to the Presidency and began a civic shift in government philosophy in the United States. With the Louisiana Purchase, puritan New England and the plantation owners of Virginia lost significant governmental power that was connected to the founding father roots of the country. Jackson's constituency was largely farmers, Irish immigrants, laborers, and settlers in the west. These groups joined to reduce the controls of the Federal Government such as Jackson's elimination of the Bank of the United States and the increase in state power.