Prior to the railroad the Mississippi Valley was potentially the basis for an independent empire, in spite of the fact that its population would inevitably be drawn from the Eastern States. Its natural outlet was down the current to the Gulf. New Orleans controlled the Valley, in the words of Wilkinson, "as the key the lock, or the citadel the outworks." So long as the Mississippi Valley was menaced, or in part controlled, by rival European states, just so long must the United States be a part of the state system of Europe, involved in its fortunes. And particularly was this the case in view of the fact that until the Union made internal commerce, based upon the Mississippi Valley, its dominant economic interest, the merchants and sailors of the northeastern States and the staple producers of the southern sea-board were a commercial appanage of Europe. The significance of the Mississippi Valley was clearly seen by Jefferson. Writing to Livingston in 1802 he declared: There is on the globe one single spot, the possessor of which is our natural and habitual enemy. It is New Orleans, through which the produce of three-eights of our territory must pass to market, and from its fertility it will ere long yield more than half of our whole produce and contain more than half of our inhabitants. . . . The day that France takes possession of New Orleans fixes the sentence which is to restrain her within her low-water mark. It seals the union of two nations who in conjunction can maintain exclusive possession of the ocean. From that moment we must marry ourselves to the British fleet and nation . . . holding the two continents of America in sequestration for the common purposes of the united British and American nations.[188:1] The acquisition of Louisiana was a recognition of the essential unity of the Mississippi Valley. The French engineer Collot reported to his government after an investigation in 1796: All the positions on the left [east] bank of the Mississippi . . . without the alliance of the Western states are far from covering Louisiana. . . . When two nations possess, one the coasts and the other the plains, the former must inevitably embark or submit. From thence I conclude that the Western States of the North American republic must unite themselves with Louisiana and form in the future one single compact nation; or else that colony to whatever power it shall belong will be conquered or devoured.