It leaves plenty of time if the confederacy only plays defense and puts all three effort into exactly that instead of the northern invasion...Why didn't anyone do that then? Maybe committing your army to do extensive field works would have effectively immobilised it and allowed for the enemy to by pass it or encircle it? There's a reason why defence in depth became a thing in WWI and WW2 when there were no flanks to turn with armies numbering in the millions and frontlines extending from coast to coast. That's not the case in the ACW where field armies had to maneuver and find each other before engaging. That doesn't leave much time to make fortifications in the defence in depth sense, and whatever field works might have been done could be made useless if the enemy army continues maneuvering instead of giving battle.
But there you say it, if you have spaced out infantry their firepower is spaced out as well, while the enemy concentrates its firepower and ability to deliver shock. Light infantry was a well known concept from the 1700s on, and if had been effective on its own then nobody would have used anything but light infantry.
And Mcclellan procrastination gives them the time..
This is obviously hypothetical since defense in depth was not invented
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