According to geographical features, the second great field of strategy lay between the Alleghany Mountains and the Mississippi River, and the third between the Mississippi River, the Rocky Mountains, and the Rio Grande.  Except in Western Virginia, the attitude of neutrality assumed by Kentucky for a considerable time delayed the definition of the military frontier and the beginning of active hostilities in the second field, thus giving greater momentary importance to conditions existing and events transpiring in Missouri, with the city of St. Louis as the principal center of the third great military field.

The same necessity which dictated the promotion of General McClellan at one bound from captain to major-general compelled a similar phenomenal promotion, not alone of officers of the regular army, but also of eminent civilians to high command and military responsibility in the immense volunteer force authorized by Congress.  Events, rather than original purpose, had brought McClellan into prominence and ranking duty; but now, by design, the President gave John C. Fremont a commission of major-general, and placed him in command of the third great military field, with headquarters at St. Louis, with the leading idea that he should organize the military strength of the Northwest, first, to hold Missouri to the Union, and, second, by a carefully prepared military expedition open the Mississippi River.  By so doing, he would sever the Confederate States, reclaim or conquer the region lying west of the great stream, and thus reduce by more than one half the territorial area of the insurrection.  Though he had been an army lieutenant, he had no experience in active war; yet the talent and energy he had displayed in Western military exploration, and the political prominence he had reached as candidate of the Republican party for President in 1856, seemed to fit him preeminently for such a duty.

While most of the volunteers from New England and the Middle States were concentrated at Washington and dependent points, the bulk of the Western regiments was, for the time being, put under the command of Fremont for present and prospective duty.  But the high hopes which the administration placed in the general were not realized.  The genius which could lead a few dozen or a few hundred Indian scouts and mountain trappers over desert plains and through the fastnesses of the Sierra Nevada, that could defy savage hostilities and outlive starvation amid imprisoning snows, failed signally before the task of animating and combining the patriotic enthusiasm of eight or ten great northwestern States, and organizing and leading an army of one hundred thousand eager volunteers in a comprehensive and decisive campaign to recover a great national highway.  From the first, Fremont failed in promptness, in foresight, in intelligent supervision and, above all, in inspiring confidence and attracting assistance and devotion.  His military administration created serious extravagance and confusion, and his personal intercourse excited the distrust and resentment of the governors and civilian officials, whose counsel and cooeperation were essential to his usefulness and success.