Siege of Petersburg
On the night of June 12, Grant again advanced by his left flank, marching to the James River. He planned to cross to the south bank of the river, bypassing Richmond, and isolate Richmond by seizing the railroad junction of Petersburg to the south. While Lee remained unaware of Grant's intentions, the Union army constructed a pontoon bridge and crossed the James River. What Lee had feared most of all was about to happen. Petersburg was the main supply base and rail depot for the entire region, including Richmond.  The taking of Petersburg by Union forces would make it impossible for Lee to continue defending Richmond.