Why doesn't the Louisiana Mississippi state line follow the Mississippi River to its mouth?

Answer 1

The parishes of Louisiana called the Florida Parishes were, indeed, once claimed by the Spanish as part of West Florida. Once Florida became part of the US, the coastal region was divided between Louisiana, Mississippi, and Alabama.

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The reason the boundary of the State of Mississippi does not follow the Mississippi River to its mouth goes back to the sequence in which the United States acquired various territories. The territory north of the coastal counties in the States of Mississippi and Alabama was part of the original United States territory ceded from England at the end of the Revolutionary War. The City of New Orleans and other Parishes east of the Mississippi River near the mouth were part of the French Louisiana Territory and were not acquired until the Louisiana Purchase under President Jefferson. This area was left as part of Louisiana.

Title to what is now the coastal counties of Mississippi and Alabama was disputed. The US argued that these counties were French territory and were included in the Louisiana Purchase. Spain argued that these territories were part of Spanish Florida. The matter was settled in 1821 when the United States acquired Florida from Spain.

When the United State organized these new territories in to States, New Orleans and nearby parishes east of the River, were left as part of the State of Louisiana. That part of Florida between the Perdido River (the western Alabama/Florida border) and the Pearl River (the lower Louisiana/Mississippi border) were cut off from the Louisiana Territory or the Florida Territory, depending on which claim was valid and made part of the Mississippi and Alabama Territories to give the access to the coastal trade.