Which American expedition was the first to cross the western portion of the United States?

The Lewis and Clark Expedition from May 1804 to September 1806, also known as the Corps of Discovery Expedition, was the first American Expedition to cross the western portion of the United States. It began in Pittsburgh, Pennsylvania, made its way westward, and passed through the Continental Divide of the Americas to reach the Pacific coast.

A selected group of US Army volunteers under the command of Captain Meriwether Lewis (1774-1809) and his close friend, 2nd Lieutenant William Clark (1770-1838) were commissioned by President Thomas Jefferson to make this Expedition shortly after the Louisiana Purchase of 1803. Their mission was to explore and to map the newly acquired territory, to find a practical route across the western half of the continent, and to establish an American presence in this territory before Britain and other European powers tried to claim it.

Additional objectives were to study the area's plants, animal life, and geography, and to establish trade with local American Indian tribes. In summary, this expedition was both a scientific and economic endeavor.

The Expedition returned to St. Louis to report its findings to Jefferson, with maps, sketches and journals in hand. They reached the Pacific Ocean, mapped and established their presence for a legal claim to the land and catalogued the various plants and animal life they encountered. They established diplomatic relations and trade with at least two dozen indigenous nations.