This whole business has incredible possibilities for changing the war.
On June 15, Baldy Smith, commanding 18th Corps begins marching toward Petersburg.Organizationally, his command is part of the Army of the James. His troops have arrived below and east of Petersburg by water. They had been attached to the Army of the Potomac during the Cold Harbor fight. In late afternoon, his troops begin collecting at the approaches to Petersburg. Smith throws a heavy skirmish line forward He discovers very strong defenses, although not well manned. Smith had a decided numerical advantage over Beauregard, who defends, but Winfield S Hancock is arriving with the 2nd Corps AoP. The two Union Corps will have an overwhelming advantage. However, Hancock, who is senior, is not sure of what his orders require, and neither have much use for a night assault. Beauregard calls in all his troops, "uncorking" Ben Butler's Army of the James at Bermuda Hundred. Beauregard is going to face at least a 4:1 numerical disadvantage at Petersburg on the 16th.
Grant arrives on the 16th, with Burnside's 9th corps coming up. To help some organizational confusion, 9th Corps is not part of either AoP or AoJ. Grant explains what he wants, and in late afternoon assaults begin. They do not go well, and darkness intervenes again.
Meanwhile, Lee is not convinced that the AoP is collecting at Petersburg. Lee has dispatched Early with two divisions and most of his cavalry west to meet a threat in the Valley. He sent Robert Hoke's division back to Beauregard, as it had been detached for Cold Harbor. Lee does not know where the AoP is, and is holding the rest of his army.
Possibilities
1) Grant "should" have taken Petersburg on the 16th or 17th, that did not happen. (2) Butler could have interdicted the direct roads and rail line between Richmond and Petersburg as he was free from Bermuda Hundred. He didn't.
If (1) or (2) happened, Richmond would be very difficult for Lee to hold..................
BUT WAIT..........................................................................
If the rail line between Richmond and Petersburg were severed no CS Marines would be going to Wilmington to commence a water borne effort on Point Lookout,MD to free POW's. And Jubal Early, in the Valley, would get no orders to cooperate with such an operation. He would have been free to march on Washington City as soon as he could. And, he would have taken Washington.
The possibilities just multiply.
As things were, Beauregard holds on by his finger nails. Early on the morning of the 18th, Lee starts sending troops to Petersburg.
Grants great opportunity is gone.
One more possibility. Lee catches on sooner. Early on the morning of the 17th, Beauregard opens up with every rifle and gun he has, a signal to Lee. Lee has moved what army he has to the west of Petersburg, and attacks on signal the Union flank. I think that would have caused great harm to the 3 Union Corps there.
If there was a blunder here, it was Smith's by not being more aggressive on the 15th. He never had a command again in the war.