"the- unaccented before a consonant, as in "the man",... before a vowel as in "the egg"...(as usually called) definite article" [or a definitive-to define]."3). Indicating uniqueness or pre-eminence, specif. a) Designating an individual or thing that has no fellow: as, the moon; the ground; the Lord;- including geographical names as, the Hudson; the Andes; the East."
"Pre-eminence- Quality or state of being pre-eminent; superiority in rank, position, excellence, etc. precedence, distinction above others in quality, rank, etc. as pre-eminence in honor." SOURCE: Webster's New International Dictionary, 2nd Edition, G&C Merriam Co., Publishers, Springfield, Mass. 1947
OPINION: Old linguistic rules can be forgotten or discarded from one scholar to another or from one historian to another creating confusion within the rank and file out in the world. In the case of "the Sudan" I would suggest to the person who originated the question to study the history of this country to discover if there was an outstanding reason for a pre-eminent designation setting it apart as it once was, whether it be geographical-terrain; political; racial; or religious, etc. I would be interested to know myself.