Henry W. Halleck, born in 1815, graduated from West Point in 1839, who, after distinguished service in the Mexican war, had been brevetted captain of Engineers, but soon afterward resigned from the army to pursue the practice of law in San Francisco, was, perhaps, the best professionally equipped officer among the number of those called by General Scott in the summer of 1861 to assume important command in the Union army. It is probable that Scott intended he should succeed himself as general-in-chief; but when he reached Washington the autumn was already late, and because of Fremont's conspicuous failure it seemed necessary to send Halleck to the Department of the Missouri, which, as reconstituted, was made to include, in addition to several northwestern States, Missouri and Arkansas, and so much of Kentucky as lay west of the Cumberland River. This change of department lines indicates the beginning of what soon became a dominant feature of military operations; namely, that instead of the vast regions lying west of the Mississippi, the great river itself, and the country lying immediately adjacent to it on either side, became the third principal field of strategy and action, under the necessity of opening and holding it as a great military and commercial highway. While the intention of the government to open the Mississippi River by a powerful expedition received additional emphasis through Halleck's appointment, that general found no immediate means adequate to the task when he assumed command at St. Louis. Fremont's regime had left the whole department in the most deplorable confusion. Halleck reported that he had no army, but, rather, a military rabble to command and for some weeks devoted himself with energy and success to bringing order out of the chaos left him by his predecessor. A large element of his difficulty lay in the fact that the population of the whole State was tainted with disloyalty to a degree which rendered Missouri less a factor in the larger questions of general army operations, than from the beginning to the end of the war a local district of bitter and relentless factional hatred and guerrilla or, as the term was constantly employed, "bushwhacking" warfare, intensified and kept alive by annual roving Confederate incursions from Arkansas and the Indian Territory in desultory summer campaigns. XIX Lincoln Directs Cooeperation--Halleck and Buell--Ulysses S. Grant--Grant's Demonstration--Victory at Mill River--Fort Henry--Fort Donelson--Buell's Tardiness--Halleck's Activity--Victory of Pea Ridge--Halleck Receives General Command--Pittsburg Landing--Island No. 10--Halleck's Corinth Campaign--Halleck's Mistakes