In cooperation with Maj. Gen. Ulysses S. Grant’s final offensive against Vicksburg, Maj. Gen. Nathaniel P. Banks’s army moved against the Confederate stronghold at Port Hudson on the Mississippi River. Like Vicksburg, Port Hudson was located atop high bluffs at the river bank that commanded the river. On May 11th, Banks learned that some Confederates had been moved from Port Hudson to support the forces defending Vicksburg, so he sought to move upon the garrison before those troops could be replaced. Banks divided his forces into two columns: one group of three divisions approaching Port Hudson from Alexandria to the north, and a second column of two divisions advancing north from Baton Rouge. By May 21st, the columns had combined and Banks's force of about 30,000 men surrounded the Port Hudson defenders and outnumbered them four to one. Banks delayed nearly a week organizing his forces, giving Maj. Gen. Franklin Gardner, in command of the Confederate defenders, time to reinforce his lines and move additional artillery into place. Banks, supported by U.S. Navy gunships, finally attacked on May 27th in a series of uncoordinated and unsuccessful attacks. Another Union attack on June 13th was beaten back, so Banks settled in for a siege. Soon the defenders exhausted their food and ammunition, and fighting and disease greatly reduced the number of men able to defend the trenches. When Gardner learned that Vicksburg had surrendered on July 4th, he realized that his situation was hopeless and surrendered on July 9th, opening the Mississippi River to Union navigation from its source to New Orleans.