Complex sentence: Washington, D.C., conterminous with District of Columbia county, which is not a part of any state, cannot be considered "independent" because it is under Congressional oversight as the U.S. national capital.
Split into simpler sentences: Washington, D.C. is coterminous with the District of Columbia, which is not a part of any state. It cannot be considered "independent" because it is under Congressional oversight as the U.S. national capital.