The Philippines and the Panama Canal Zone: Both were United States territories but never part of a state. Also, the Guano Islands.
Retroceded Lands Not Part of States
|From the State of Maine|
Tensions rose nearly to the point of war in 1838 but a treaty dividing the area more favorably to the British secured peace has held since. In the end all the United States loss was a few ethnically French villages in southeastern Quebec.
Montana's Louisiana Purchase
|Lost Northern Montana. From Wikipedia|
The reason for this is geopolitical. The 1783 Treaty of Paris gave the United States lands east of the Mississippi River and south of the Rainy River. However, the Mississippi River does not meet up with the Rainy River and this would leave a gap of present-day Minnesota, North Dakota, and South Dakota British. So the Treaty of 1818 gave the Red River Basin of the Dakota and Minnesota to the United States in exchange for established 49 degrees North as the border between the United States and Canada. This chopped of a bit of what would have become northern Montana.
The United States gained land on which Fargo, Grand Banks, Bismark, and Bemidji. In exchange it gave of land which is now includes the dark sky preserve of Grasslands National Park and very few people.
Texas may be big but it was once bigger. The reason why Texas is now smaller is because of the Big River, the Rio Grande. Rivers may seem like a great idea for a border, they form culturally and political borders so easily. However, one must always remember rivers can and do move and that each physical geography move can create problems with human geography.
The 1848 Treaty of Guadalupe Hidalgo defined the American Texas-Mexico border as the Rio Grande. The river kept meandering though to the point that in the late 1800s an island in the river was split by the river while merging into the north and south banks. This created the odd problem of part of Mexico being north of the Rio Grande and part of the United States being south of the river.
|Transfer of land between the United States and Mexico. From Wikipedia|
Today the lost land is now a park dedicated to cooperation between the United States and Mexico.
Rio Rico is a small hamlet towards the end of the Rio Grande as it approaches the Gulf of Mexico. In 1906 a private company doing irrigation work in Texas diverted part of the Rio Grande which caused the river to move and cut off Rio Rico from the rest of Texas. Mexican authorities moved in and the small town de facto became part of Mexico despite being still legally a part of Texas. In a somewhat surprising act of apathy no one seemed to care. Rio Rico was purely Hispanic, very small in population with less than 1,000 people, and the border between the United States and Mexico was very open back then (one could merely cross the river with no border or customs enforcement for the most part). In the 1920s and 1930s the mob set up a presence in Rio Rico as a spot in the United States where one could drink alcohol.
Eventually it was decided that everything should become formal on paper. The Boundary Treaty of 1970 established that Rio Rico would officially become part of Mexico in 1977. After some court cases U.S. federal court ruled that all people born in Rio Rico born between 1906 and 1977 were legally United States citizens. Thousands upon thousands of Mexicans all the sudden declared they were born in Rio Rico. After some verification processes Rio Rico emptied out as the residents were allowed U.S. citizenship.
Today Rio Rico is a quiet town in the Mexican state of Tamaulipas. Note the image below. One can still tell where the old border was.
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