Johnston began to move his force west to join Pemberton when he heard of that general's defeat at Champion Hill (May 16) and Big Black River Bridge (May 17). The survivors retreated to the fortifications of Vicksburg. Johnston urged Pemberton to avoid being surrounded by abandoning the city and to join forces with Johnston's troops, outnumbering Grant, but Davis had ordered Pemberton to defend the city as his highest priority. Grant launched two unsuccessful assaults against the fortifications and then settled in for a siege. The soldiers and civilians in the surrounded city waited in vain for Johnston's small force to come to their rescue. By late May Johnston had accumulated about 24,000 men but wanted additional reinforcements before moving forward. He considered ordering Bragg to send these reinforcements, but was concerned that this could result in the loss of Tennessee. He also bickered with President Davis about whether the order sending him to Mississippi could be construed as removing him from theater command; historian Steven E. Woodworth judges that Johnston "willfully misconstrued" his orders out of resentment of Davis's interference. Pemberton's army surrendered on July 4, 1863. Along with the capture of Port Hudson a week later, the loss of Vicksburg gave the Union complete control of the Mississippi River and cut the Confederacy in two. President Davis wryly ascribed the strategic defeat to a "want of provisions inside and a general outside [Johnston] who would not fight."