Adams–Onís Treaty
The Third Treaty of San Ildefonso ceded to France the vast territory that Napoleon then sold to the United States in 1803, known as the Louisiana Purchase. The United States obtained Spanish Florida in 1819 in the Adams–Onís Treaty. That treaty also defined a northern border for New Spain, at 42° north latitude (now the northern boundary of the U.S. states of California, Nevada, and Utah).