New France was a large area of North America colonized by France, beginning in 1534.
The French built dozens of forts across their massive territory for protection from Native Americans, and from the expanding influence of Britain in the Americas.
By the middle of the 18th century, over 70,000 people inhabited New France, but Britain's colonies along the Atlantic Ocean coastline exceeded one million.
During the French and Indian War (1754–1763), British forces defeated the French and their Indian allies; shortly thereafter the American Revolution began in Britain's original thirteen colonies. The British lost that war and the Treaty of Versailles (1784) (on paper) awarded much of this land to the upstart United States.
In 1800, when the armies of French ruler Napoleon Bonaparte moved across Europe, pressuring Spain into a corner, this huge slice of land in the middle of America, and Spain's expansive territorial holdings at that time in the western mountain areas, (a collective 827,987 sq miles in size) were ceded back to France by Spain via the (secret) Treaty of San Ildefonso.
In 1803, Napoleon Bonaparte sold all of that land including the French Territory of Louisiana to the U.S. for $15 million dollars, in a transaction called the Louisiana Purchase. Spain ceded (lost) all of its remaining territories within the borders of modern-day America, beginning in Florida in 1819.