"Now, general, I think I am even with you," continued Grant, in his quiet, unimpressible manner; and the reverend general was obliged to own that he had been flanked in his own manoeuvre. Columbus was the western extreme of the rebel line of defence, which at that time included nearly the whole length of the Potomac River in the East. The enemy had built Fort Henry on the Tennessee River, and Fort Donelson on the Cumberland, and occupied Bowling Green, near the centre of the State of Kentucky. The line which included these points was the boundary which actually separated the territories in possession of the combatants. It was the strategic line of the rebels, on which they had placed their defences, concentrated their armies, and gathered their supplies, both for aggressive movements to the North, and to prevent a Union force from penetrating to the South. Bowling Green, on a branch of the Green River, was at the junction of the two lines of railroad from Memphis and from Nashville. Fort Henry and Fort Donelson were near the Memphis road, protecting it from Union raiders, and supplied by it with men and provisions, as well as by the two rivers. The Gibraltar of the West, as Columbus was called by the rebels, mounted one hundred and forty guns, was abundantly supplied with men and material, and its railway connections afforded every facility for reënforcing it in case of necessity. It closed the Mississippi against the Union steamers and gunboats. Fort Henry, the first connecting link in the rebel line of defence, was a strongly-built fortification on the right bank of the Tennessee, mounting seventeen guns, and provided with accommodations for fifteen thousand men. Fort Donelson, on the left bank of the Cumberland, was a more elaborate work, mounting forty guns, and with quarters for twenty thousand troops. These two forts effectually closed the rivers on which they were located, and were only twelve miles apart, so that they could coöperate with each other in cases of emergency. A strong rebel army at Bowling Green completed the defence, and an advance by land was as impracticable as by water. The problem which the Western military commanders were called upon to solve was, how to break through this line. The question seems to have worried Grant to no inconsiderable degree, and he studied the matter attentively during the winter. In January, by order of General Halleck, he sent out a heavy force under General Smith, in the direction of Columbus, to aid a movement on the part of General Thomas in another part of the state, and to examine the ground. No fighting was done, and the soldiers suffered severely from cold; but the object of the expedition was gained, for Thomas defeated the rebels at Mill Springs, where the result would have been different if reënforcements had been sent from this quarter to the enemy.