While it might seem more obvious that the Union would want to take control of the Mississippi — and of course they did — the Tennessee and Cumberland had strategic priority. First, they both flow northward rather than southward. This means that when a Union gunboat is attacking from the north and is disabled, it drifts back north, out of range of the guns, rather than south directly into them, as it would on the Mississippi. Second, the Tennessee goes right down through Tennessee and then along the Mississippi-Alabama border, and then into northern Alabama. It was navigable halfway across Alabama, to Muscle Shoals. The Cumberland River is only about 10 miles east of the Tennessee at the Kentucky/Tennessee border, but then diverges eastward along much of Tennessee, providing access to the capital, Nashville. Controlling these rivers would give the Union a couple of highways into the heart of the South.
During Grant’s diversionary reconnaissance south into Kentucky, C.F. Smith had reported that Fort Henry on the Tennessee was highly vulnerable to gunboat attack. Halleck ordered Grant to go ahead and capture the fort.
From the Official Record:
HEADQUARTERS DEPARTMENT OF THE MISSOURI,
Saint Louis, January 30, 1862.
Major General GEORGE B. McCLELLAN,
General-in-Chief of the Army, Washington:
GENERAL: I inclose herewith a copy of instructions sent this day to General Grant in relation to the expedition up the Tennessee River against Fort Henry. As Fort Henry, Dover [Fort Donelson], &c., are in Tennessee, I respectfully suggest that that State be added to this department.
General Grant has already been re-enforced with eight regiments of infantry, and several others, with three batteries of artillery, are under orders to join him. I will send down every man I can spare. Information is received to-day that Brigadier-General Price, son of the major-general, is again organizing insurgents in Howard and Chariton Counties, and that the rebels are becoming more hold since our troops have been sent south. I therefore think it unsafe to withdraw many more until the State militia can take their place. The militia dare not or will not organize in counties not occupied by our troops. To facilitate this organization it becomes necessary to scatter the volunteers over a very large tract of country. This is unfortunate, but unavoidable.
Fort Henry has a garrison of about 6,000, and is pretty strongly fortified. Possibly re-enforcements may be sent from Columbus as soon as we move. If we can reach the railroad this may be prevented, as the country roads are almost impassable.
The troops from Rolla are advancing in the direction of Springfield, but necessarily move very slowly. Greenville, south of Ironton, is occupied by our cavalry, and an infantry regiment is ordered to re-enforce them. This movement is necessary to break up the rebel organizations in the counties of Wayne and Butler.
The roads south of the Tennessee River are almost impassable. General Smith reported on his recent reconnaissance up that river “that the road was horrible, and new tracks had to be cut through the woods. It took an entire day for one brigade to move 3 miles.”
Permanent crews for the gunboats are being rapidly organized. The mortar boats cannot be used in the Tennessee or Cumberland, and I doubt if they will ever be of much use in the Mississippi. Neither navy nor army officers have much faith in them.
Very respectfully, your obedient servant,
H. W. HALLECK,