Grant fixed the fourth of May for the combined advance of all the converging forces of invasion. There were two weak points where the Union armies failed: one in the farthest south, where, as we have so often seen, Banks could not attack Mobile owing to his absence at Red River; the other in the farthest north, where Sigel was badly beaten and replaced by Hunter. Here, after much disabling interference at the hands of Stanton, Hunter was succeeded by Sheridan, whom Grant himself directed with consummate skill. There were also two Confederate thorns in the Federal side: Forrest's cavalry in Sherman's rear, Mosby's cavalry in Grant's. Forrest roved about the river area, snapping up small garrisons, cutting communications, and doing a good deal of damage right up to the Ohio. Mosby, with a much smaller but equally efficient force, actually raided to and fro in Grant's immediate rear; and on one occasion nearly captured Grant himself just on the eve of the opening move. As Grant's unguarded special train from Washington pulled up at Warrenton Junction, where there was only one Union official, Mosby's men had just crossed the track in pursuit of some Federal cavalry. But neither these two Confederate thorns in the side nor the more serious Federal failures could stop the general advance. Nor yet could Butler's lack of success on the James. Butler had seized and fortified an exceedingly strong defensive position at Bermuda Hundred on a peninsula, with navigable water on both flanks and in rear, and a very narrow neck of land in front. The only trouble was that it was as hard for him to surmount the Confederate front across the same narrow neck as it was for the enemy to surmount his own. He was, in fact, bottled up, with the cork in the enemy's hands. He did send out cavalry from Suffolk to cut the rails south of Petersburg. But no permanent damage was done there. Petersburg itself, which at that time was almost defenseless, was not taken. And in the middle of the month Beauregard attacked Butler so vigorously as to make the Army of the James rather a passive than an active force till it was presently, absorbed by Grant when he arrived before Richmond in June.