Considerable skirmishing had preceded the Union approach, as the Confederate pickets fell slowly back inside the defenses, thus preventing a close inspection of the Confederate fortifications. Grant determined, however, to attack immediately, before Pemberton had time to post his defenses strongly. The Union general ordered an assault at 2 p. m. on the 19th. Sherman’s troops, whose early arrival had enabled them better to prepare for attack, moved under heavy fire against the Confederate left. Although they succeeded in making a close lodgment against the walls of the Stockade Redan, they failed to breach the works and were repulsed. McPherson and McClernand, not yet in good position for attack, were unable to do more than advance several hundred yards closer to the siege line. Grant lost 1,000 men testing the Vicksburg defenses and discovered an unyielding army manning the works. Confederate losses were slight. [Illustration: _Confederate Railroad Redoubt. Plaques mark angle where fort wall was breached and entered by Union troops during the assault of May 22, 1863._] THE ASSAULT OF MAY 22. While the probing operation of the 19th had failed, Grant further considered the important results which a successful assault would achieve. Such a move, however costly, would save a long siege. In the end, fewer men might be lost, and a growing threat to the Union rear—General Johnston raising troops near Jackson for the relief of Vicksburg—could be eliminated by quickly capturing Vicksburg and throwing the entire Union strength against Johnston. In addition, the Federal troops, spirited by recent victories and impatient to seize the prize for which they had campaigned so long, would not work so zealously in the trenches with pick and shovel unless assault had failed. On the 21st, Grant issued orders for a general assault against Vicksburg the following day. [Illustration: _The heavy guns of this Union siege battery were borrowed from the Federal gunboats and used against the Confederate siege defenses._ From _Photographic History of the Civil War_.] The Union assault of May 22 was delivered against the center of the Confederate siege line along a 3-mile front from Stockade Redan to Fort Garrott. The felled trees and thick undergrowth, as well as the precipitous faces of the ravines, restricted the scope of Union maneuver. Only a portion of Grant’s full strength could be brought into action, reserves being posted to exploit a breakthrough. Careful preparations preceded the attack: field batteries were run forward and emplaced to provide a covering fire for the infantry, and troops were advanced into concealed positions—in places, within 200 yards of their objective. Watches of all Union commanders were synchronized, the attack to begin simultaneously at 10 a. m. in order to prevent Pemberton from shifting his forces from one threatened point to another.