And this great crisis was only the bursting into flame of many smaller fires that had long been smoldering. For generations the two sections had been drifting apart. Since the middle of the seventeenth century, Mason and Dixon's line had been a line of real division separating two inherently distinct portions of the country. By 1860, then, war was inevitable. Naturally, the conflict would at once present intricate military problems, and among them the retention of the Pacific Coast was of the deepest concern to the Union. Situated at a distance of nearly two thousand miles from the Missouri river which was then the nation's western frontier, this intervening space comprised trackless plains, almost impenetrable ranges of snow-capped mountains, and parched alkali deserts. And besides these barriers of nature which lay between the West coast and the settled eastern half of the country, there were many fierce tribes of savages who were usually on the alert to oppose the movements of the white race through their dominions. California, even then, was the jewel of the Pacific. Having a considerable population, great natural wealth, and unsurpassed climate and fertility, she was jealously desired by both the North and the South. To the South, the acquisition of California meant enhanced prestige--involving, as it would, the occupation of a large area whose soils and climate might encourage the perpetuation of slavery; it meant a rich possession which would afford her a strategic base for waging war against her northern foe; it meant a romantic field in which opportunity might be given to organize an allied republic of the Pacific, a power which would, perchance, forcibly absorb the entire Southwest and a large section of Northern Mexico. By thus creating counter forces the South would effectively block the Federal Government on the western half of the continent. The North also desired the prestige that would come from holding California as well as the material strength inherent in the state's valuable resources. Moreover to hold this region would give the North a base of operations to check her opponent in any campaign of aggression in the far West, should the South presume such an attempt. And the possession of California would also offer to the North the very best means of protecting the Western frontier, one of the Union's most vulnerable points of attack. It was with such vital conditions that the Pony Express was identified; it was in retaining California for the Union, and in helping incidentally to preserve the Union, that the Express became an important factor in American history.