William H. Seward's detractors believed that the Alaska Purchase was a great mistake, calling it Seward's Folly. 

In 1846, the Oregon Treaty fixed the U.S. border with Canada at the 49th parallel.

The Battle of Little Bighorn was the most decisive victory by Native American tribes against the United States Army. General George Armstrong Custer led his 7th Cavalry across the northern Great Plains throughout early 1876, trying to corral various tribes onto reservations.  On June 25, Custer came upon a large united force of Lakota, Cheyenne, and Arapaho warriors, and the 7th Cavalry was decimated. Custer and all of his officers were killed.  In the aftermath of Little Bighorn, the US Army greatly expanded its efforts against Native American tribes, and began a more concentrated effort to relocate tribes to reservations.

The Mexican-American War was controversially started after the United States annexed the Republic of Texas in 1845. Despite large swaths of opposition, the U.S. Army quickly dominated the Mexican Army. The U.S. conquered present day New Mexico, Arizona, and California, and then proceeded all the way to Mexico City. The Army's commanders, Zachary Taylor and Winfield Scott, quickly became national heroes, and were given the nicknames of "Old Rough and Ready" and "Old Fuss and Feathers." Taylor would be elected President in 1848, and died in office in 1850. Scott was commander of the Armed Forces until the Civil War.

After hostilities began in 1861, the Union Armies found limited success in the theaters in the War. Fighting at this stage was largely confined to the area of Northern Virginia between the capitals of Richmond and Washington in the East, and along the broader Mississippi and Ohio River Valleys in the Western theater. The only place with significant Union victories was along the Mississippi and Ohio River Valleys.

In 1867, the Russian Empire was convinced that it would lose its possessions in America due to problems stemming from the Crimean War. Anxious to recoup some value, the Russian Czar negotiated a treaty with US Secretary of State William Seward. The United States, in what critics called Seward's Folly, bought what would become the state of Alaska for $7.2 million.

The Indian Removal Act of 1830 was a large part of Andrew Jackson's efforts to clear out more land in the South for white settlers who were increasingly moving into lands claimed by Native Americans. While Jackson claimed he could take the land as the executive, the Supreme Court overruled him. Nonetheless, Jackson still moved the Cherokee and Creek Tribes to Oklahoma Territory in what is known as the Trail of Tears.

The War of 1812 was seen as a source of national pride for the young American nation--and a sideline to the serious business of the Napoleonic Wars by the British. Most of the initial causes of the war for the Americans directly related to British conduct during the Napoleonic Wars, including detaining sailors heading to French ports, seizing goods from American ships, and violating American territorial claims in Canada.

The Gadsden Purchase acquired nearly 30,000 square miles of the southern portion of the present day states of New Mexico and Arizona, settling territorial disputes stemming from the Treaty of Guadalupe Hidalgo in 1846. The largest backers of the Gadsden Purchase were Southern political leaders, who saw the land as an opportunity to build a transcontinental railroad from the Southern states to California. While the purchase was successful, the railroad never got built across the land.

During 1876, the U.S. pursued a policy of forcing Native American tribes on the Great Plains onto reservations, which was widely opposed by many groups. The Army sent a large Cavalry force, led by General George Armstrong Custer and accompanied by scouts from the Crow tribe, to seek out a combined force of Arapaho, Northern Cheyenne, and Lakota Sioux in Wyoming and Montana. On June 25, 1876, the two armies met at the Little Bighorn River in southern Montana, where an overwhelming force led by the Sioux chief Sitting Bull destroyed Custer's troops, killing the General and much of his staff.