James Knox Polk was born on November 2, 1795 in rural Mecklenburg County, North Carolina where his Irish family had moved from Pennsylvania. The oldest of ten children, Polk had always been groomed to take over the family farm but he proved too sickly to handle the work. In 1806, his family moved to Tennessee. At sixteen, he had to be operated on to remove gallstones. Eventually, his parents began his education at age seventeen by sending him off to a private academy and then on to the University of North Carolina, where he graduated second in his class.

Polk studied law under the celebrated Tennessee jurist Felix Grundy before opening his own law office. The Panic of 1819, caused by the malfunctioning Second Bank of the United States, hit the state hard. That year, Polk began his political career by being selected as the clerk of the state legislature. In the capital, he met Sarah Childress, the daughter of a prominent family, and they married in 1824. They never had children.

In 1823, he was elected to the state legislature in his own right and began to support Tennessee native Gen. Andrew Jackson for the presidency. The Democrat Jackson lost the election in a vote in the House of Representatives, however, after failing to win a majority in the Electoral College. Polk, elected to the House himself in 1825, quickly became one of Jackson's strongest allies in Congress and helped lead the fight against internal improvements and to lower tariffs. Largely because of the success of the Democrats, Jackson overwhelmingly won the election in 1828 and Polk found himself rising quickly in the ranks of Congress. Polk helped Jackson fight against the rechartering of the National Bank and got promoted to the chairmanship of the House Ways and Means committee for his efforts. He also developed a reputation for cordiality, respect and logical, well-thought-out arguments. As Jackson stepped down and Jackson's protégé Martin Van Buren won the presidency, Polk won election as Speaker of the House. He presided over some of the stormiest sessions of Congress ever recorded as the issue of slavery began to heat up.

In 1839, Polk stepped down as speaker and ran for governor of Tennessee at Jackson's request, to try and keep Tennessee a Democratic state. He lost reelection in 1841 and immediately began running for the 1843 election. However, along the way, he turned down a Senate appointment by the state legislature and the controversy eventually prevented Tennessee from electing any senators that year. The bitterness from Tennessee's lack of representation in Congress kept Polk from winning the governor's election in 1843.

The following year, though, Polk's strong support of the annexation of Texas–one of the year's most controversial issues–and his further insistence that America expand northward to the 54th parallel made him an attractive "Dark Horse" presidential candidate to the Democratic party. He won the nomination unanimously on the ninth ballot after the Democrats indicated their unwillingness to re-nominate Van Buren.

Polk edged out Henry Clay in the fall election and, once he assumed office, began the single most ambitious plan of expansion ever executed by a president. Using slogans like "Fifty-Four, Forty or Fight" and "Manifest Destiny," Polk moved ahead with plans to annex Texas, Oregon, New Mexico and California.

Luckily, England was unwilling to go to war over the Oregon territory and Polk successfully negotiated a compromise on the 49th parallel (the current U.S.–Canada border). However, after initial attempts at a diplomatic or a financial solution to the problem of Texas failed, he sent Zachary Taylor and an army of three thousand into the disputed territory. The Mexican War, which would last for most of Polk's term, demonstrated the problems of contemporary communication systems, as Polk found himself dealing with politically ambitious generals with whom he would lose contact for months at a time. The generals often directly disobeyed his orders and, in the end, the peace treaty was signed for the U.S. by a renegade State Department clerk with no authority to sign a treaty. In the treaty, however, Polk got almost everything he wanted: Texas and the rights to purchase California and New Mexico.

Moreover, as other European countries agitated to become involved in politics in the Western Hemisphere, Polk firmly restated the Monroe Doctrine, which forbade foreign involvement in the Americas.

Polk also had an ambitious domestic agenda, and again, what it is remarkable was his ability to achieve so much of it. He reformed the postal service, lowered tariffs, and pushed Jeffersonian principles throughout his term. He also struggled to settle the question of slavery in the new territories that he had acquired, but for the most part the slavery question would wait until the Civil War.

Polk retired after only one term in office. He had almost worked himself to death and could not and did not want to serve another term. He was one of the most successful presidents of the 1800s and certainly one of the strongest. He died just a few months after leaving office, unable to regain his strength from his long, hard years as president.