I guess I'm asking "how necessary was the Civil War?"
Once the first Southern states seceded, why did the Union go on the offensive?
I'm approaching this question from the "War of Northern Aggression" perspective here. Is there a good reason why the Union went to war rather than just let the South do it's thing via secession and the Confederacy.
Needless to say, I know about the idealistic reasons. But those usually aren't the REAL reason why nations go to war.
What is the basis for your assumption that the Union went on the offensive?
The general consensus is that the Confederacy struck first in its assault on Ft. Sumter. There are some claims that the first shots of the war were fired in Florida -- but these too involve Confederate attacks on Union installations or personnel.
Now you could ask "why did the North not withdraw from its southern installations as soon as Southern states demanded it and immediately recognize secession rather than holding onto them and trying to forestall secession through the Crittenden compromise, Washington Peace Conference, etc?"
Or you could possibly even ask "Why did the Union respond to Confederate attacks rather than just suing for peace immediately?
However, asking about Union motives requires keeping in mind the actual sequence of events that lead to the outbreak of the Civil War.
UPDATE: To add to the incidents of Ft. Sumter and Ft. Pickens, there was also the Star of the West incident as a possible "first shots of the Civil War" (essentially happening at the same time as the Ft. Pickens incidents in Pensecola) -- but this was also a case of Confederates firing a Union ship (an unarmed supply vessel, incidentally).
Great answers to this in the FAQ
Most sources I've seen from the North mostly journals indicate a desire to preserve the Union. There was a real fear that if the southern states seceded the United states would be broken. Other states would be able to secede at will and the divided nation could be swallowed up by European Powers.