Grant's crossing of the James altered his original strategy of attempting to drive directly on Richmond, and led to the Siege of Petersburg. After Lee learned that Grant had crossed the James, his worst fear was about to be realized—that he would be forced into a siege in defense of the Confederate capital. Petersburg, a prosperous city of 18,000, was a supply center for Richmond, given its strategic location just south of the capital, its site on the Appomattox River that provided navigable access to the James River, and its role as a major crossroads and junction for five railroads. Since 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 the Confederate capital. This represented a change of strategy from that of Grant's Overland Campaign, in which confronting and defeating Lee's army in the open was the primary goal. Now, Grant selected a geographic and political target and knew that his superior resources could besiege Lee there, pin him down, and either starve him into submission or lure him out for a decisive battle. Lee at first believed that Grant's main target was Richmond and devoted only minimal troops under Gen. P.G.T. Beauregard to the defense of Petersburg as the siege of Petersburg began.