If I ask the question: where are you going? You could answer "To the United States" or "To America"; "To the United Kingdom" or "To Britain"; "To the Eiffel Tower" or "To Paris". I think the rule is this: only if the name has a common noun (such as "states", "kingdom", "tower", "ocean", "river", etc) then it will be preceded by "the". You may ask ... what about "The Atlantic" or "The Amazon", etc. Well, I think those are just shortened forms of "The Atlantic Ocean" and "The Amazon River". So, "the" is required if the name has a common noun, even if the common noun is dropped/omitted.