On 12 May, part of the army was defeated by McPherson’s corps of Grant’s army, at Raymond, just twelve miles west of Jackson. On 13 May, that force had moved back to Jackson, where Johnston found them on his arrival that evening.
There was some uncertainty about the number of men that Johnston had at Jackson. Early on 14 May McPherson wrote to Sherman, telling him that Johnston had arrived in the state capitol, and was said to have 20,000 men, although McPherson doubted he had that many. After the battle, Johnston wrote to Pemberton, telling him that he had 6,000 men that he was free to move. The force at Jackson included at least 4,000 men forced out of Raymond, twelve miles to the west (12 May). They had been joined by another thousand on 13 May. Johnston reported finding 6,000 men in Jackson when he arrived on 13 May, with another 5,000 on their way. However, when he became aware of the approach of McPherson and Sherman, Johnston ordered those reinforcements to stop.
After the fighting at Raymond, Grant had decided to deal with the forces at Jackson. He ordered two of his three army corps, Sherman’s and Jackson’s, to carry out that attack. Meanwhile, Johnston had decided to evacuate Jackson. His 6,000 men were sent to slow down the advancing Federals, to give him time to get the precious supplies out of the city. They succeeded in this. The Federal attack began at 11.00 a.m., and Jackson was in their hands by 4 p.m. They had lost 42 dead, 251 wounded and 7 missing, and inflicted about 800 casualties on the Confederate defenders.
The true significance of the battle and occupation of Jackson was that it prevented Johnston from using the town as a base for his rapidly expanding army. By the time Vicksburg surrendered, that force had expanded to 24,000 men, but when it mattered those reinforcements were scattered and unable to influence events. Grant was free to turn west to deal with Pemberton, who had now brought a large part of his army out of Vicksburg. The two would clash two days later at Champion’s Hill.