- 1803: Louisiana Purchase doubles the size of the United States.
- 1812-1815: War of 1812 with Great Britain.
- 1820: Missouri Compromise admits Missouri as a slave state and Maine as a free state, and sets the boundary between the two in the Louisiana Territory.
- 1846-1848: Mexican-American War.
- 1848: Gold Rush begins in California.
- 1850: Compromise of 1850 admits California as a free state, establishes the territories of New Mexico and Utah, and strengthens the Fugitive Slave Act.
1850-1900
- 1861-1865: American Civil War.
- 1862: Homestead Act gives public land to settlers.
- 1863: Emancipation Proclamation frees slaves in the Confederate states.
- 1865: Thirteenth Amendment abolishes slavery in the United States.
- 1868: Fourteenth Amendment grants citizenship to all people born or naturalized in the United States.
- 1870: Fifteenth Amendment guarantees the right to vote to all male citizens regardless of race.
- 1890: Sherman Antitrust Act breaks up monopolies.
- 1895: Plessy v. Ferguson upholds the concept of "separate but equal" public facilities.
- 1898: Spanish-American War.
- 1900: Women's suffrage movement gathers momentum.