In 1800, when the armies of French ruler Napoleon Bonaparte moved across Europe, pressuring Spain into a corner, the Territory of Louisiana (New Orleans) and a huge slice of land controlled by Spain in the now central United States (an estimated 827,987 sq miles in size) was ceded back to France by Spain via the Treaty of San Ildefonso.
On April 1, 1803, the United States of America was comprised of lands east of the Mississippi River, including a handful of states (17), and the Illinois, Indiana, Michigan and Mississippi Territories.
At that time, besides the French Territory of Louisiana; modern-day Florida was still governed by Spain, and in the far west, excluding the British-controlled Oregon Country in the northwest, all available lands were still in the hands of Spain.
In 1801, U.S. President Thomas Jefferson began his pursuit of just the French City of New Orleans (French Orleans Territory) in an effort to gain control of Mississippi River access; his $10 million offer was initially rejected by the French.
Over the next year Napoleon was actively involved in his growing European military campaign against Britain, as well as other political conflicts in the Americas; apparently stretched-thin, he tired of his Louisiana Territory (and needing focus and funds) he finally wavered, and on April 30, 1803 he authorized the sale of the entire French Territory of Louisiana for $15 million, which included the City of New Orleans; justifiably, President Jefferson was jubilant.
The nation-changing Louisiana Purchase subsequently almost doubled the size of the United States overnight. This massive transfer of land included all of present-day Arkansas, Oklahoma, Missouri, Kansas, Nebraska and Iowa; most of Colorado, South Dakota, Montana and Wyoming, as well as significant parts of North Dakota, Minnesota, Texas and Louisiana.
Parts of it also extended into what is now Canada for a short period of time. Eventually Spain ceded (lost) all of its territories to the United States, beginning in Florida in 1819, as the modern-shape of the U.S.A. finally came together.