What: France sells Louisiana territories to USA for $15 Million ($342 Million in today’s money).

When: 1801-1806.

Where:  The newly acquired Louisiana Territory, in the USA.

Why: The Louisiana Purchase was the first major cession of land in a long series of expansions that span the 19th century.

What: Meriwether Lewis and William Clark lead an expedition to explore and map the newly acquired Louisiana Territory.

When: 1804-1806.

Where: West of the Mississippi River

Why: Lewis and Clark's team mapped uncharted land, rivers, and mountains.

What: The Oregon Trail was a key part of the westward expansion of the United States. It facilitated the settlement of the Oregon Territory and later contributed to the idea of Manifest Destiny

When: In the 1830s and 1840s.

Where: The trail began in Independence, Missouri, and stretched over 2,000 miles to the Willamette Valley in Oregon.

Why: It contributed to the idea of Manifest Destiny—the belief that the U.S. was destined to expand across the North American continent.

What: Thousands of Cherokee people were forced to leave their homes and undertake a grueling journey westward. The harsh conditions, lack of proper provisions, and exposure to the elements led to the deaths of around 4,000 Cherokee, nearly a quarter of the tribe's population.

When: 1830-1850.

Where:  Southeastern states of the actual US.

Why: President Andrew Jackson forced removal and migration of many eastern Indian nations to lands west of the Mississippi and kill 4,000 of the Cherokee people.

What: Congress authorized Jones to call for an Annexation Convention.

When: 1845

Where: Texas

Why: The annexation of Texas contributed to the coming of the Mexican-American War (1846-1848). The conflict started, in part, over a disagreement about which river was Mexico's true northern border: the Nueces or the Rio Grande.

What: A skirmish near the Rio Grande between U.S. and Mexican forces sparks the U.S. declaration of war on Mexico.

When: 1836-1848

Where: In Mexico and the U.S.

Why: Mexico recognize the Rio Grande as Texas's border, cedes California and the present-day southwestern states to the U.S., and receives $15 million in return.

What: It formalized the border between the United States and British North America west of the Rocky Mountains.

When: 1818-1846.

Where: Washington D.C.

Why: The Oregon Treaty of 1846 resolved the western border between the United States and British Canada. It created what would later be the US states of Oregon and Washington (and parts of Idaho, Montana, and Wyoming) and the Canadian province of British Columbia.

What: In 1849, the Gold Rush peaked with around 80,000 "Forty-Niners" arriving in California, boosting the population to about 100,000 by year's end.

When: 1849-1850

Where: U.S. California

Why: Discovery of gold in California leads to a massive influx of settlers, contributing to rapid population growth in the region.

What: President Abraham Lincoln signed the Homestead Act on May 20, 1862. On January 1, 1863, Daniel Freeman made the first claim under the Act, which gave citizens or future citizens up to 160 acres of public land provided they live on it, improve it, and pay a small registration fee.

When: May 20, 1862

Where: Beatrice, Nebraska.

Why: To help develop the American West and spur economic growth, Congress passed the Homestead Act of 1862, which provided 160 acres of federal land to anyone who agreed to farm the land.

What: The First Transcontinental Railroad is completed, linking the East and West coasts and facilitating easier travel and trade.

When: 1869-1890.

Where: Sacramento, California.

Why: The U.S. Census Bureau declares that the American frontier has closed, symbolizing the end of westward expansion.