The Mississippi formed no barrier whatsoever to the march of the backwoodsmen. It could be crossed at any point; and the same rapid current which made it a matter of extreme difficulty for any power at the mouth of the stream to send reinforcements up against the current would have greatly facilitated the movements of the Ohio, Kentucky, and Tennessee levies down-stream to attack the Spanish provinces. In the days of sails and oars a great river with rapid current might vitally affect military operations if these depended upon sending flotillas up or down stream. But such a river has never proved a serious barrier against a vigorous and aggressive race, where it lies between two peoples, so that the aggressors have merely to cross it. It offers no such shield as is afforded by a high mountain range. The Mississippi served as a convenient line of demarkation between the Americans and the Spaniards; but it offered no protection whatever to the Spaniards against the Americans. Importance of New Orleans. Therefore the frontiersmen found nothing serious to bar their farther march westward; the diminutive Spanish garrisons in the little creole towns near the Missouri were far less capable of effective resistance than were most of the Indian tribes whom the Americans were brushing out of their path. Towards the South the situation was different. The Floridas were shielded by the great Indian confederacies of the Creeks and Choctaws, whose strength was as yet unbroken. What was much more important, the mouth of the Mississippi was commanded by the important seaport of New Orleans, which was accessible to fleets, which could readily be garrisoned by water, and which was the capital of a region that by backwoods standards passed for well settled. New Orleans by its position was absolute master of the foreign, trade of the Mississippi valley; and any power in command of the seas could easily keep it strongly garrisoned. The vast region that was then known as Upper Louisiana--the territory stretching from the Mississippi to the Pacific--was owned by the Spaniards, but only in shadowy fashion, and could not have been held by any European power against the sturdy westward pressure of the rifle-bearing settlers. But New Orleans and its neighborhood were held even by the Spaniards in good earnest; while a stronger power, once in possession, could with difficulty have been dislodged. Desire of the Settlers for it.