This period marked the beginning of major emigration to the far West.

1840:
The expression "O.K." comes into use. A campaign slogan, it alludes to Martin Van Buren's nickname, "Old Kinderhook". Kinderhook, New York, is Van Buren's birthplace. The sixth census: U.S. population - 17,069,453.

1841:
William Henry Harrison is inaugurated as the ninth President of the United States; Harrison dies one month later. John Tyler is inaugurated as the tenth President of the United States. Tyler is the first Vice President to succeed to the office by the death of a President. The Pre-Emption Act of 1841 gives "squatters" the right to purchase federal land, acquired at a minimum price.

1842:
The migration of white settlers to the Oregon Country, begins primarily from the Ohio Valley region and Missouri. The main route is the Oregon Trail, which begins at Independence, Missouri, and terminates at Oregon City and the Willamette Valley.

1843:
Joseph Smith, leader of the Mormon Church, announces that a divine revelation has sanctioned the practice of polygamy.

1844:
The first long-distance telegraph message, "What hath God wrought?" is sent from the U.S. Supreme Court room in Washington, D.C., to Baltimore, Maryland, by Samuel F.B. Morse.

1845:
James K. Polk is inaugurated as the eleventh President of the United States.

1846:
American soldiers provoke an attack by the Mexican Army in disputed Texas Territory, and the U. S. declares war on Mexico. Brigham Young, successor to the murdered Mormon founder and leader Joseph Smith, organizes the Westward migration of Mormons. The exodus is started by anti-Mormon terror in Nauvoo, Illinois. A severe potato famine in Ireland precipitates large-scale emigration to the U.S.

1847:
After a campaign of several months, General Winfield Scott enters Mexico City, and the war with Mexico is brought to an end. A battalion of U.S. Marines begins guarding the "Halls of Montezuma". Brigham Young, with an advanced party of 148, reaches the Valley of the Great Salt Lake. Cyrus McCormick begins manufacture of his reaper in Chicago, a farm implement which will revolutionize U.S. and European agriculture in the years to follow.

1848:
Gold is discovered at Sutter's Mill near Sacramento, California. A women's rights convention is held at Seneca Falls, New York and inaugurates the modern feminist movement, with a resolution on women's rights prepared under the leadership of Lucretia Mott and Elizabeth Cady Stanton. The main body of Mormon migrants reach the Great Salt Lake and begin building Salt Lake City. The Mormons proclaim the "State of Deseret."

1849:
Zachary Taylor is inaugurated as the twelfth President of the United States. The Pacific Railroad Company is chartered, and constructs the first railroad west of the Mississippi River, from St. Louis to Kansas City.
Civil Disobedience by Henry David Thoreau is published.

1850:
President Zachary Taylor dies suddenly of an acute intestinal infection. Millard Fillmore is inaugurated as the thirteenth President of the United States. Fifty-five thousand emigrants move west along the Oregon Trail, most bound for the gold fields of California. Levi Strauss creates the first pair of "bibless" overalls in California. Seventh Census: U.S. population - 23,191,867.

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