In his first coordinated offensive, Scott showed a Jominian streak that would take quite a long thread to explore. There was also visible a level of daring and complexity reaped from long experience. Here, in late spring of 1861, Scott makes the overland campaigns of later years look stupid by comparison.
At the end of his first offensive, the main point had been gained with the threat of a combination against inferior forces. Dangerous movements on multiple axes had compelled the enemy's decision to evacuate. Some movements succeeded, some failed; enough were put into motion that the failures might not influence a positive outcome.
Scott had stocked his main column with extra forces to enable it to fight a major battle to gain Harpers Ferry. Another force fought its way across what is now Western Virginia near to the decision point. Another force fought through Maryland to the crossing for Leesburg, Virginia. Alexandria was occupied. A push out of Ft. Monroe was defeated while a push from Washington fizzled.
The campaign might make a nice example in Jominian study, with forces operating on exterior lines for military advantage. Perhaps a misunderstanding of this campaign "inspired" Lincoln's misguided 2/22/62 order.
Let's try now to pass from the Jominian to the Clausewitzian. Scott did not know Clausewitz but we can still do some analysis. What political objective or purpose did the campaign serve?
The taking of Harpers Ferry closed off the Cumberland Valley, securing Patterson's Department of Pennsylvania, making Harrisburg safe, Gov. Curtin calmer, and gaining control of high profile federal property at a railroad chokepoint. It also opened the possibility of additional major military developments.
But politically, it was "low yield."
Let's consider the other potential objectives in this campaign from the political standpoint.
Grafton - The fall of Grafton unlocks western Virginia for the establishment of a loyal Virginia, dealing serious political damage to the secessionist state and general Rebel government.
Suffolk - The defeat at Big Bethel forestalls this. Its total potential, politically, might have been to spread fear and demorilization in the enemy capital. This would have been the maximum political upside.
Centreville - McDowell could not organize his demonstration toward Centreville in time to help Patterson. Had Centreville fallen in the course of that or had it been abandoned without a battle, the benefit would have been to force retirement of the provocatively named enemy "Army of the Potomac," transforming it into an army of the Rapahannock.
Edward's Ferry - The political effect of Stone's victories was to secure another state government - Maryland - from embarassment by roving Confederate forces.
As mentioned yesterday, Beatie discusses in some detail the next steps considered by Scott and Patterson after this first campaign, especially their idea of advancing far enough down the Valley to place an army behind (under, south of) Richmond, thereby forcing evacuation of the South's Potomac positions.
Place yourself in Scott's shoes, considering your next land offensive. The highest value project available, politically, is the forcing of an evacuation of the Potomac line. Defensively, the highest political priority is protection of the capital which implies the collection of all available forces there, rather than at HF or Ft. Monroe.
The means available to gain the largest political prize are indirect - a Valley approach by Patterson, McClellan, and maybe Stone - or direct - an assault on Centreville from Washington, Centreville being the nexus for supply and reinforcement of the Potomac line.
The table has been set by political (Clausewitzian) priority; Jominian principles then dictate that the force collected around Washington should be applied against the foe closer to home on shorter (exterior) lines with less distance (less risk) from the Union base of operations (DC).
And so, thanks to theory, there is a cetain predictability to Centreville becoming the principal target in Scott's second coordinated offensive of 1861.