The history of the United States from 1815 to 1849—also called the Middle Period, the Antebellum Era, or the Age of Jackson—involved westward expansion across the American continent, the proliferation of suffrage to nearly all white men, and the rise of the Second Party System of politics between Democrats and Whigs. Whigs—representing merchants, financiers, professionals, and a growing middle class—wanted to modernize society, using tariffs and federally funded internal improvements; Jacksonian Democrats opposed them and closed down the national bank in the 1830s. The Jacksonians favored expansion across the continent, known as manifest destiny, dispossessing Native Americans of lands to be occupied by farmers, planters, and slaveholders. As a result of the annexation of Texas, the defeat of Mexico in war, and a compromise with Britain, the western third of the nation rounded out the continental United States by 1848.

The transformation America underwent was not so much political democratization but rather the explosive growth of technologies and networks of infrastructure and communication, including with the telegraph, railroads, the post office, and an expanding print industry. These developments made possible the religious revivals of the Second Great Awakening, the expansion of education, and social reform. They modernized party politics and sped up business by enabling the fast, efficient movement of goods, money, and people across an expanding nation. They transformed a loose-knit collection of parochial agricultural communities into a powerful cosmopolitan nation. Economic modernization proceeded rapidly, thanks to highly profitable cotton crops in the South, new textile and machine-making industries in the Northeast, and a fast developing transportation infrastructure.

Breaking loose from European models, the Americans developed their own high culture, notably in literature and in higher education. The Second Great Awakening brought revivals across the country, forming new denominations and greatly increasing church membership, especially among Methodists and Baptists. By the 1840s, increasing numbers of immigrants were arriving from Europe, especially British, Irish, and Germans. Many settled in the cities, which were starting to emerge as a major factor in the economy and society. The Whigs had warned that annexation of Texas would lead to a crisis over slavery, and they were proven right by the turmoil of the 1850s that led to the Civil War.

Names

Historians of the United States have referred to this approximate period in American history using various names. It has been called the Middle Period. According to Trevor Burnard in 2011, the era "used to be called the Middle Period". This span of years has also been called the Antebellum Era or Antebellum Period. The word antebellum comes from the Latin ante bellum, meaning "before the war", and the age preceded the Civil War era.

Another name for the time period has been the Age of Jackson or Jacksonian era, named after American president Andrew Jackson, said to have defined the era as a time when popular democracy became the United States' norm and ideal. Historian Daniel Walker Howe criticized this name, arguing that although Jackson used democratic rhetoric, his presidency was authoritarian and did not broadly represent the country.

Era of Good Feelings and the rise of nationalism

After the War of 1812, the United States began to assert a newfound sense of nationalism. America began to rally around national heroes like Andrew Jackson. Patriotic feelings were aroused by Francis Scott Key's poem The Star-Spangled Banner. Under the direction of Chief Justice John Marshall, the Supreme Court issued a series of opinions reinforcing the role of the national government. These decisions included McCulloch v. Maryland and Gibbons v. Ogden, both of which reaffirmed the supremacy of the national government over the states. The signing of the Adams–Onís Treaty helped settle the western border of the country through popular and peaceable means.

Sectionalism Even as nationalism increased across the country, its effects were limited by a renewed sense of sectionalism. The New England states that had opposed the War of 1812 felt an increasing decline in political power with the demise of the Federalist Party. This loss was tempered with the arrival of a new industrial movement and increased demands for northern banking. The Industrial Revolution in the United States was advanced by the immigration of Samuel Slater from Great Britain and arrival of textile mills beginning in Lowell, Massachusetts. In the south, the invention of the cotton gin by Eli Whitney radically increased the value of slave labor. The export of southern cotton was now the predominant export of the U.S. The western states continued to thrive under the "frontier spirit." Individualism was prized as exemplified by Davey Crockett and James Fenimore Cooper's folk hero Natty Bumppo from Leatherstocking Tales. Following the death of Tecumseh in 1813, Native Americans lacked the unity to stop white settlement.

Era of Good Feelings Domestically, the presidency of James Monroe (1817–1825) was hailed at the time and since as the "Era of Good Feelings" because of the decline of partisan politics and heated rhetoric after the war. The Federalist Party faded away, but without an opponent the Democratic-Republican Party decayed as well, since the sectional interests came to the fore.

The Monroe Doctrine was drafted by Secretary of State John Quincy Adams in collaboration with the British, and proclaimed by Monroe in late 1823. He asserted the Americas should be free from additional European colonization and free from European interference in sovereign countries' affairs. It further stated the United States' intention to stay neutral in wars between European powers and their colonies but to consider any new colonies or interference with independent countries in the Americas as hostile acts towards the United States. No new colonies were ever formed.

Annexation of Florida and border treaties As the 19th century dawned, Florida had been undisputed Spanish territory for almost 250 years, aside from 20 years of British control between the French and Indian Wars and the American Revolution. Although a sparsely inhabited swampland, expansionist-minded Americans were eager to grab it and already, in 1808, American settlers had invaded the westernmost tip of Florida and expelled the local Spanish authorities, after which Congress hastily passed a bill annexing it under the claim that the Louisiana Purchase had guaranteed the territory to the United States. During the War of 1812, American troops occupied and seized the area around Mobile Bay. Spain, then engulfed in war with France, did not react to either of these actions. Also taking advantage of the mother country's distraction, Spain's Latin American colonies rose up in revolt and Madrid was forced to denude Florida of troops to suppress the rebellions. As the Spanish withdrew, Native American and pirate raids from Florida to the US increased. In 1818, Andrew Jackson led an army into Florida to quell the chaotic situation there. He arrested and hanged two British agents who had been encouraging Indian raids, leading to an outcry in London and calls for war. However, cooler heads prevailed and the situation did not escalate further. A year later, Secretary of State John Quincy Adams negotiated the Adams–Onís Treaty with Spain. The Spanish agreed to turn over the no-longer-defensible Florida to the US and also give up their extremely flimsy claims to the distant Oregon Territory, in exchange for which American claims on Texas were renounced (some Americans had also been claiming parts of that territory under the Louisiana Purchase). The heretofore vague border between the US and Spanish North America was also settled upon. Although American designs on Texas did not disappear, they were put on the backburner for the more immediately important Florida.

Meanwhile, in 1818, the U.S. and Britain also agreed to settle the western border with Canada, which was established at the 49th parallel running straight from the Great Lakes to the Rocky Mountains. Included in this settlement was the headwaters of the Red River in what would eventually become Minnesota, and the Mesabi Range, which eventually proved to contain vast amounts of iron ore. The eastern border of Canada continued to be disputed and was not settled until 1845.

Emergence of Second Party System

Monroe was re-elected without opposition in 1820, and the old caucus system for selecting Republican candidates soon collapsed into factions. In the presidential election of 1824, factions in Tennessee and Pennsylvania put forth Andrew Jackson. From Kentucky came Speaker of the House Henry Clay, while Massachusetts produced Secretary of State Adams; a rump congressional caucus put forward Treasury Secretary William H. Crawford. Personality and sectional allegiance played important roles in determining the outcome of the election. Adams won the electoral votes from New England and most of New York; Clay won his western base of Kentucky, Ohio, and Missouri; Jackson won his base in the Southeast, and plus Illinois, Indiana, North Carolina, South Carolina, Pennsylvania, Maryland, and New Jersey; and Crawford won his base in the South, Virginia, Georgia, and Delaware. No candidate gained a majority in the Electoral College, so the president was selected by the House of Representatives, where Clay was the most influential figure. In return for Clay's support in winning the presidency, John Quincy Adams appointed Clay as secretary of state in what Jacksonians denounced as a corrupt bargain.

During Adams' administration, new party alignments appeared. Adams' followers took the name of "National Republicans", to reflect the mainstream of Jeffersonian Republicanism. Elected with less than 35% of the popular vote, Adams was a minority president and his cold, aloof personality did not win many friends. Adams was also a poor politician and alienated potential political allies with his commitment to principles when he refused to remove Federal officeholders and replace them with supporters out of patronage. A strong nationalist, he called for the construction of national road networks and canals and renewed George Washington's call for a national academy. Adams even went so far as to suggest the construction of astronomical observatories to rival those of Europe. These extravagant proposals offended many average Americans. Southerners in particular opposed them because they would require continued heavy tariffs, and they feared government overreach of this type could easily lead to action taken against slavery. Adams had little to boast about in domestic affairs.

Despite his sterling career in foreign affairs, Adams also proved less than successful at foreign policy. His old rival, British Foreign Secretary George Canning, played a cat-and-mouse game with him. Ever since the Treaty of Paris 42 years earlier, Britain had barred American merchantmen from doing business in its islands in the West Indies, although smugglers frequently evaded this ban. When Adams demanded that London open the islands to trade, Canning rejected his request. Another fiasco for the president occurred when the newly independent Latin American republics held a congress in Panama in 1826. Adams requested permission and funding from Congress to send two delegates. Some Congressmen worried about becoming involved in foreign entanglements, while Southerners, sensitive to racial issues, disliked the idea of granting recognition and equal status to the "black" and "mixed" Latin American states. Although Adams was ultimately successful in getting approval, one of the two delegates died en route to Panama and the Panama Congress ultimately accomplished little of value.

Jacksonian democracy