John G. Nicolay, A Short Life of Abraham Lincoln, condensed from Nicolay and Hayes' Abraham Lincoln: A History

This left Vicksburg as the single barrier to the complete opening of the Mississippi, and that barrier was defended by only six batteries and a garrison of six Confederate regiments at the date of Farragut's arrival before it. But Farragut had with his expedition only two regiments of troops, and the rebel batteries were situated at such an elevation that the guns of the Union fleet could not be raised sufficiently to silence them. Neither help nor promise of help came from Halleck's army, and Farragut could therefore do nothing but turn his vessels down stream and return to New Orleans. There, about June I, he received news from the Navy Department that the administration was exceedingly anxious to have the Mississippi opened; and this time, taking with him Porter's mortar flotilla and three thousand troops, he again proceeded up the river, and a second time reached Vicksburg on June 25.

The delay, however, had enabled the Confederates greatly to strengthen the fortifications and the garrison of the city. Neither a bombardment from Porter's mortar sloops, nor the running of Farragut's ships past the batteries, where they were joined by the Union gunboat flotilla from above, sufficed to bring the Confederates to a surrender. Farragut estimated that a cooperating land force of twelve to fifteen thousand would have enabled him to take the works; and Halleck, on June 28 and July 3, partially promised early assistance. But on July 14 he reported definitely that it would be impossible for him to render the expected aid. Under these circumstances, the Navy Department ordered Farragut back to New Orleans, lest his ships of deep draft should be detained in the river by the rapidly falling water. The capture of Vicksburg was postponed for a whole year, and the early transfer of Halleck to Washington changed the current of Western campaigns.