Delaware.
The Sun Belt spans the southern USA from Florida to California. California is the furthest west Sun Belt state.
Iowa's southern border is with the state of Missouri.
The US state of South Dakota is the southern border of North Dakota.
what state makes up most of arkansas' southern border
Arkansas
Virginia
At the beginning of the US Civil War, the "southern" border States were Kentucky and Missouri. In this answer, Maryland is excluded in that although it was a pro=southern slave State, it cannot be considered a border State.
The Ohio River
It is Missouri.
Kansas is the US state on the southern border of Nebraska. The capital city of Kansas is Topeka.
Oklahoma's southern border borders Texas. Oklahoma City is the capital and largest city in the U.S. state of Oklahoma.