One generation after the Declaration of Independence launched the democratic American experiment, the young nation appeared to be thriving in 1840. The United States population had surged to 17 million, an increase of nearly 33% since 1830. America’s population even surpassed that of England, with just over 15 million people, and Spain, with 14 million.

But America was just getting started. Throughout the 1840s, the nation expanded not only its population but also its territory. The annexation of Texas added about 390,000 square miles to the United States, while the subsequent Mexican-American War provided another 525,000 square miles — an immense addition of land that dramatically reshaped the nation’s future.

Americans continued to push west of the Mississippi River in search of opportunity. Some followed the emerging Oregon Trail. Others made the arduous trek to Utah under the leadership of Mormon pioneer Brigham Young. The idea of “Manifest Destiny” — the belief that the United States was destined to stretch from coast to coast — gained widespread traction and was fueled by the discovery of gold in California in 1848.

Yet with this rapid expansion came rising tensions. The question of whether slavery would be permitted in newly acquired territories deeply divided Americans and inched the nation toward the Civil War.

Despite the momentum of growth and opportunity, most Americans in 1840 still lived on small farms. But that way of life was beginning to change as new technologies and inventions accelerated the growth of factories and cities, drawing workers from rural America and newly arriving immigrants from overseas.

Meanwhile, social movements challenged long-standing norms. Women’s rights advocates held their first national convention, abolitionists including Frederick Douglass gained influence, and new voices emerged in literature, science and the arts.

Headlines of the 1840s revealed both extraordinary progress and deepening tensions that would help shape the United States into the nation it would become.

The 1840s were one of only two decades in U.S. history in which five presidents served (the other came in the 1880s).

The political merry-go-round began in 1840 when President Martin Van Buren lost his reelection bid in a landslide to Indian fighter and military hero William Henry Harrison. At age 68, Harrison was the oldest person to assume the presidency at that time. He delivered the longest inaugural address in U.S. history — an astonishing 1 hour and 45 minutes — while standing hatless and coatless on a cold, wet March day. He died of pneumonia 30 days later, becoming both the first president to die in office and the president with the shortest tenure.

The Alexandria (Virginia) Gazette published the somber announcement on April 6, 1841, signed by Secretary of State Daniel Webster and three other cabinet members:

“An all-wise Providence having suddenly removed from this life, WILLIAM HENRY HARRISON, late President of the United States, we have thought it our duty, in the recess of Congress, and in the absence of the Vice President from the Seat of Government, to make this afflicting bereavement known to the country, by this declaration, under our hands.

He died at the President’s House, in this city, this fourth day of April, Anno Domini, 1841, at thirty minutes before one o’clock in the morning.”

Vice President John Tyler succeeded Harrison as the nation’s 10th president and served the remainder of the term.

In 1844, Democrat James Polk won the presidency. At just 49, he was the youngest man to assume the office to that point. During his single term, America expanded by more than one-third and reached the West Coast. Polk kept his campaign promise to serve only one term and died of cholera only four months after leaving office.

The final president to take office during the decade was Mexican-American War hero Zachary Taylor, elected in 1848. He became the fifth president to occupy the White House during the 1840s. Taylor died of an illness in July 1850, becoming the second president within 10 years to die in office.

Americans had been pressing westward since the colonial era, but the movement accelerated in the 1840s.

In 1841, the first emigrant wagon train set out from Missouri toward California, traveling 1,730 grueling miles. Along the way, about half the 70 men, women and children in the party split off and headed to Oregon instead. What began as a trickle soon became a torrent. Nearly 10,000 emigrants followed the Oregon Trail in the decade, including the ill-fated Donner Party.

Among the most notable emigrants was Brigham Young, who led thousands of Mormons to Utah in 1846 and 1847. Over the next two decades more than 60,000 Mormons would complete this journey.

But it was the discovery of gold at Sutter’s Mill in California in January 1848 that triggered the largest mass migration in U.S. history. Roughly 100,000 prospectors and fortune seekers arrived in California the following year. These newcomers became known as the Forty-Niners.

When the 1840s began, California was still a distant northern province of Mexico. But the annexation of Texas in 1845 — a former Mexican territory that had won independence in 1836 — heightened tensions between the two nations. When Mexico refused to sell California and New Mexico, the U.S. sent troops into the disputed border region. After a controversial clash, Congress declared war in May 1846.

The conflict became a training ground for future Civil War leaders such as Ulysses S. Grant, Robert E. Lee and Thomas “Stonewall” Jackson. Abraham Lincoln, then a first-term congressman, spoke out forcefully against the war, questioning its justification.

After a series of costly American victories, the war ended with the Treaty of Guadalupe Hidalgo in 1848. For $15 million, Mexico ceded 55% of its territory — including present-day California, Nevada, Utah, much of Arizona and New Mexico, and parts of Colorado and Wyoming. With this acquisition, the United States added 525,000 square miles and finally stretched from the Atlantic to the Pacific.

The 1840s saw significant cultural and social change as new voices and movements emerged nationwide.

In July 1848, the first women’s rights convention was held in Seneca Falls, N.Y. About 300 people — both women and men — attended the meeting to discuss Elizabeth Cady Stanton’s Declaration of Sentiments. Modeled after the Declaration of Independence, it laid out the grievances women faced, including their inability to vote, their lack of property rights, their limited access to education, and the difficulty of obtaining a divorce. One hundred attendees signed the document, including former slave and abolitionist Frederick Douglass.

This decade also produced enduring works of literature and music. Edgar Allan Poe published “The Raven” in 1845, captivating audiences with its haunting rhythm. Pittsburgh-born Stephen Foster wrote “Oh, Susanna” in 1848, widely considered the first major hit in American popular music.

In 1845, Frederick Douglass released “Narrative of the Life of Frederick Douglass, an American Slave,” a groundbreaking autobiography that galvanized the abolitionist movement.

Horace Greeley founded the New York Tribune in 1841, a Whig newspaper promoting reform, abolitionism, literature and advocacy for the masses. Greeley rejected the sensationalism that dominated many New York papers, which he condemned as a “horrid medley of profanity, ribaldry, blasphemy, and indecency.” By the Civil War, the Tribune had over 250,000 subscribers and Greeley was regarded as the most influential editor in the nation.

As America grew physically, philanthropy began to shape its landscape as well. In 1842, a scandalous personal story led to what may be the nation’s first major land donation.

Mary Croghan, heiress to a vast Pittsburgh fortune, caused a stir when she eloped at a young age with much older English sea captain Edward W. Harrington Schenley. The Pittsburgh Mercury and Democrat reprinted this sensational account from the New York Express on Feb. 16, 1842:

“A young Miss, in her 13th year, from the West, perhaps the richest heiress in the whole West, whose property, too, was in her own right, recently left this city for Liverpool with a man about sixty. The young lady left by her father at a boarding school near this city, where the gentleman became acquainted with her, and it is believed that a marriage ceremony was performed before they left.”

Mary and Edward Schenley went on to have nine children. Whether she reconciled with her father remains unclear, but in her will she donated 300 acres to the City of Pittsburgh, forming the foundation of present-day Schenley Park.

While America lost one young heiress to an English marriage, it gained well over a million new residents from Europe. The Great Famine in Ireland, beginning in 1845, devastated the potato crop that sustained much of the population. An estimated 1 million people died of starvation between 1845 and 1849, and between 1.5 and 2 million emigrated — many to the United States.

German immigration surged as well. Crop failures, economic pressures and a series of political revolutions prompted thousands to seek a new start in America during the 1840s.

The 1840s included technological advancements that transformed communication, industry and everyday life.

On May 24, 1844, Samuel Morse sent the first telegraph message — “What hath God wrought” — from Washington, D.C., to Baltimore. The telegraph revolutionized business, journalism, transportation and eventually warfare during the Civil War.

Other technological breakthroughs rapidly followed. In 1844, Charles Goodyear patented the vulcanization process for rubber. Elias Howe patented the lock stitch sewing machine in 1846, while Walter Hunt filed for the safety pin patent in 1849. Samuel Slocum patented the stapler in 1841. The first adhesive postage stamps, bearing the images of George Washington and Benjamin Franklin, went on sale in 1847.

Photography also advanced. In 1844, Matthew Brady opened his first portrait studio in New York City. His subjects included Edgar Allan Poe, Daniel Webster, James Fenimore Cooper, and Zachary Taylor — one of 18 presidents Brady photographed during his career.

As photography developed, the nation’s favorite pastime was just emerging.

In the early 1840s, Alexander Cartwright and his friends formed the New York Knickerbockers, the first organized baseball club. They developed rules that replaced earlier informal versions of the game, establishing nine innings, marking foul territory and standardizing team sizes at 8-11 players. These rules laid the foundation for modern baseball.

Boxing also captured public attention. On Sept. 9, 1841, Tom Hyer defeated George McChester in 101 rounds to become the first American heavyweight boxing champion. The match lasted 2 hours and 55 minutes.

Not all bouts had triumphant endings. On Sept. 26, 1842, the Signal of Liberty newspaper reported that fighter Thomas McCoy had died following a 77-round match in New York. “The details of this brutal affair are too disgusting for our columns,” it reported.

The decade saw numerous natural and human-made disasters.

The Great Natchez Tornado in May 1840 killed 317 people and injured 109, making it the second-deadliest tornado in U.S. history. Mount St. Helens erupted on Nov. 22, 1842, long before its more famous 1980 eruption.

Steamboats, while technologically advanced, were notoriously dangerous. The steamship Lexington burned and sank off Long Island in 1840, killing 139 of the 143 passengers and crew. In 1841, the paddle wheel steamer Erie caught fire near Buffalo, killing 242 people. In 1847, the steamer Phoenix was destroyed on Lake Michigan with an estimated 200 lives lost.

On land, the Donner Party became a grim symbol of the dangers of westward expansion. The 87-member wagon train attempted a shortcut to California in 1846, only to be trapped by snow in the Sierra Nevada for four months. Forty died, and survivors resorted to cannibalism. Rescue parties finally reached the remaining 48 in the spring of 1847.

The era also marked major achievements in exploration.

Lt. Charles Wilkes led a U.S. Navy expedition from 1838 to 1842 that circumnavigated the globe, surveyed the Pacific and mapped the Antarctic coastline. His team charted 1,500 miles and was the first to recognize Antarctica as a continent.

Meanwhile, explorer John C. Fremont, accompanied by scout Kit Carson, mapped vast stretches of the American West between 1841 and 1846. He helped chart the Oregon Trail, explored the Sierra Nevada, and is believed to be the first white man to view Lake Tahoe. He also confirmed that the Great Basin had no outlet to the sea.

Fremont later played a military role in California during the Mexican-American War and became one of the state’s first U.S. senators. In 1856, he ran for president as the first nominee of the new Republican Party but lost to Democrat James Buchanan.

Several enduring American institutions were born in the 1840s.

On Aug. 10, 1846, Congress officially established the Smithsonian Institution, funded by the estate of British scientist James Smithson. Though Smithson had never visited the United States, he bequeathed his fortune to create an organization dedicated to “the increase and diffusion of knowledge.” Today, the Smithsonian is the world’s largest museum, education and research complex.

On March 3, 1849, Congress created the U.S. Department of the Interior to manage domestic affairs, including exploration of Western lands, oversight of territorial governments, public park management and relations with Native American tribes.

The 1840s were a turning point in the American story. They were years when the country grew faster, pushed farther and argued louder than ever before. New lands and new technologies opened possibilities that earlier generations could hardly imagine. At the same time, hard questions about slavery, rights, citizenship and national identity came to the surface.

It was a decade of wagon trains creaking toward the Pacific, telegraph wires humming for the first time, factories rising, and reform movements finding their voices.

But the same forces that energized the nation also exposed its deepest fractures. Every mile of westward expansion raised urgent debates about who would control this growing country, and at what cost. It was a decade that didn’t just precede the upheavals of the 1850s and the Civil War, it helped set them in motion and shaped the nation that would emerge in the decades to come.