Final answer:
The Lewis and Clark Expedition began their exploration of the Louisiana Territory in May 1804. They prepared for the expedition at the mouth of the Missouri River during the winter of 1803-1804 after Jefferson's authorization in 1803.
Step-by-step explanation:
Lewis and Clark Expedition Departure
The Lewis and Clark Expedition, commissioned by Thomas Jefferson, began in May of 1804. This mission of exploration, led by Captain Meriwether Lewis and Lieutenant William Clark, was aimed at exploring the Louisiana Territory. Despite being mentioned in excerpts from a letter dated 20 June 1803, the actual departure of the expedition occurred after spending the winter of 1803-1804 preparing at the mouth of the Missouri River.
Their journey was significant for its contributions to the United States' geographical and scientific knowledge, as well as the expansion of American influence and potential trade routes. When they set off in May 1804, they faced numerous challenges and made detailed journals about the geography, the native tribes, and the wildlife they encountered. Lewis was later appointed governor of the Louisiana Territory, but the mission itself was a critical step in the nation's westward expansion.