What does this sentence mean?(HELP DON'T UNDERSTAND)?

The North controlled the southern end of the Mississippi from Baton Rouge/New Orleans south to the Gulf of Mexico. They also controlled the northern end of the river from Minn. to Memphis after the capture of Fort Pillow made Memphis impossible for the South to Hold. The only section that the South still controlled was the area around Vicksburg Mississippi. The North started operations to take Vicksburg and cut the confederacy in 2.