Final answer:
Thomas Jefferson was concerned about the American use of Spanish-held New Orleans because it was a vital port and granting access to the Mississippi River. The loss of access to the port led to outrage and calls for war. Jefferson recognized the importance of New Orleans in facilitating westward expansion and trade.
Step-by-step explanation:
Thomas Jefferson was concerned about the American use of Spanish-held New Orleans because it was a vital port for the United States and granting access to the Mississippi River. When Spain ceded Louisiana to France in 1801, the United States lost its right to deposit goods free in the port, leading to outrage and calls for war. Jefferson recognized the importance of New Orleans in facilitating westward expansion and trade with other countries. He instructed Robert Livingston and James Monroe to secure access to New Orleans, and the timing proved advantageous as Napoleon was motivated to sell Louisiana after losing control over Haiti. The Louisiana Purchase was essentially concluded in 1803, doubling the size of the United States.