Final answer:
The most important achievement of Jefferson's first administration was the acquisition of the Louisiana Territory.
Step-by-step explanation:
The most important achievement of Jefferson's first administration was the acquisition of the Louisiana Territory. This significant event occurred in 1803 when the United States purchased a vast stretch of land from France, doubling the size of the country. By acquiring this territory, Jefferson expanded American influence and secured future opportunities for westward expansion and economic growth.
Learn more about Jefferson's first administration and the acquisition of the Louisiana Territory