James Knox Polk was the right president at the right time. He came to power amid a growing sense of "Manifest Destiny" that encouraged westward expansion and imparted a sense of duty to those moving toward the ever-growing frontiers. Perhaps, then, it is fitting that Polk grew up, for much of his life, on the frontier–then defined as Tennessee.
In 1795, when America was born, the country was still struggling with how to live under its new government. The Constitution had only been adopted six years prior, and Polk was only just beginning his schooling when the War of 1812 began. The whole political process–and the whole nation–was still being shaped.
Polk entered politics just as political parties began to exercise great power. Andrew Jackson's Democratic Party, based largely on Jeffersonian principles of democracy, had arisen and now opposed the Whig Party, led by John Quincy Adams and Henry Clay. The Whigs favored higher tariffs and internal improvements. For his part, Jackson retired as the most popular president since George Washington and Polk so closely aligned himself with and learned so much from Jackson, whose nickname was "Old Hickory," that Polk earned the nickname "Young Hickory."
Polk's expansionist ideas grew out of the same sense that pervaded American thinking in the late 1830s and 1840s. "Manifest Destiny" held that the U.S. had an almost divine right to expand across the continent and bring its freedoms to as many as possible. Texas, an equally independent area, seceded from Mexico in 1835 and formed its own country–much to the consternation of both the U.S. and Mexico. When Polk earned the Democratic nomination for the Presidency it was largely because he was one of the only politicians to forcefully support the annexation of Texas as a territory of the United States.
The issue of slavery, though, was the great divide during Polk's career. The Founders of the American Constitution had believed that the issue would slowly wither away and die of its own accord, that there was no need to deal with it in a long-term way. However, the southern plantation owners believed that their "peculiar institution" was vital to the production and processing of cotton. As the northern states slowly became more industrial they outlawed slavery, creating a growing disconnect between slave and free states. Although the issue came up many times in Polk's political career, it would not fully erupt until a decade after his death–when the election of Abraham Lincoln and the Civil War split the nation in two.