Union General Nathaniel Banks was determined to invade East Texas via the Red River through Louisiana. Earlier in the war, Banks had seen Galveston, Texas seized but retaken by the Confederates in January 1863 and had then been repulsed at Sabine Pass in September 1863.
While successful invading South Texas in November 1863, Banks once again focused his efforts at a fourth invasion of Confederate Texas. In truth, Union General Henry Halleck had consistently urged Banks to undertake the Red River campaign; Banks preferred attacking Galveston from the sea rather than overland from Louisiana.
Since Grant would have preferred Banks moving against Mobile, Alabama, the Red River campaign is one of the reasons Halleck was demoted when Grant was promoted to the position of general in chief of all Union armies.