James K. Polk was the first president since John Quincy Adams to win the office without a majority of the popular vote.¹⁰

The size of the United States doubled with the Louisiana Purchase (1803) and doubled again (1819–1848) with the acquisition of Florida, Texas, Oregon Territory, and the northern half of Mexico. So, elderly Americans living in 1850 had seen the physical extent of their country quadrupled within their lifetimes. If that rate of expansion had continued until the present, the United States would today cover every square inch of land on the planet, and then some.¹¹

By the Treaty of Guadalupe Hidalgo, which ended the Mexican-American War, the United States acquired an area of more than 500,000 square miles, including lands that now comprise California, Nevada, and Utah, most of New Mexico and Arizona, and parts of Wyoming and Colorado. Next to the Louisiana Purchase, Guadalupe Hidalgo marked the largest single addition to the national domain, and the price paid by the United States to Mexico for the land—$15 million—exactly matched that paid for Louisiana 45 years earlier.¹²

At least one in four Texans was a slave in 1850, five years after the state's annexation into the United States.¹³

Fewer than 60,000 white Americans resided west of Mississippi River in 1845. The West was considered a foreign wilderness by most Americans—hundreds of thousands of Native Americans of over 200 different tribes made their homes there. But white settlement increased rapidly thereafter. More than 4.3 million Americans had crossed the great river and moved westward by the start of the Civil War.¹⁴

Although it was constitutionally required, Congress never formally passed a declaration of war on Mexico in 1846. A spending bill authorizing the mobilization of forces for the battle contained a preamble that declared "a state of war exists" between Mexico and the United States due to "the act of the Republic of Mexico."¹⁵

Tales of the California Gold Rush, particularly at the very end of the war in 1848 and 1849, incited a combination of greed and adventurousness—both quite characteristic of the antebellum frontier climate—that contributed to a desertion rate among American soldiers of about 8%, which remains the highest of any American war.¹⁶