Adam Badeau, Military history of Ulysses S. Grant from April 1861 to April 1865. Volume 1

While the investment of Vicksburg was thus proceeding, the menacing attitude of Johnston had early attracted Grant's attention, and made it necessary to establish a strong corps of observation in the rear. Immediately after the assault of the 22d of May, the small cavalry force attached to the command was sent out, to interrupt the enemy's communications, and to obtain accurate information as to the movements of the relieving force. It was soon learned that Johnston had been joined by at least ten thousand fresh troops; and Grant was thus made reasonably certain that the rebels would endeavor to raise the siege, attacking from the northeast, with all the men they could command. He, therefore, wrote to Banks, on the 25th: ‘I feel that my force is abundantly strong to hold the enemy where he is, or to whip him if he should come out. The place is so strongly fortified, however, that it cannot be taken without either a great sacrifice of life, or by a regular siege. I have determined to adopt the latter course, and save my men. . . . . The great danger now to be apprehended is that the enemy may collect a force outside, and attempt to rescue the garrison. . . . I deem it advisable that as large a force be collected here as possible. Having all my available force that can be spared from West Tennessee and Helena here, to get any more I must look outside of my own department. You being engaged in the same enterprise, I am compelled to ask you to give me such assistance as may be in your power. . . . I would be pleased, general, to have you come, with such force as you may be able to spare.’