In fact, the North was reluctant to go to war-- the South did fire first on Fort Sumter and by definition, the Confederate States seized a range of American soil against the will not just of the North, but against the will of many Southerners, especially in the border states. Remember that West Virginia seceded from Virginia in the course of the war.
Suppressing succession, even without slavery as an issue, is not "imperialism."
But here is the crux of the issue-- stopping secession and holding the Union together was obviously the strongest initial impulse of the US Civil War, but anti-slavery was unquestionably more than a subtext but a prime motivation for not just "the public" but among the national leadership in the North.
I specifically would argue that if you look at the economic views of the Radical Republicans, you will find such a diversity of pro- and anti-corporate views that the only issue holding them together was anti-slavery. And when the common denominator is anti-slavery goals, then it is reasonable to see that goal as the prime issue, not the subtext.
It more reasonable to see that many corporate types took advantage of that prime goal to serve their own interests, but the corporate exploiters of the Civil War were the tail, not the dog. Most of the big winners of the Civil War did not start out rich or powerful, so they were not in a position to drive policy. They were merely opportunists who took advantage of the situation to enrich themselves and end up powerful AFTER the war.
Pro-corporate power in the North was largely a by-product of the Civil War, not its cause. So you are reversing causality.
BTW it's worth noting that most of the leaders in the North driving the Civil War were decidely against the US imperialism that had led to the Mexican-American War and was pushing for imperial adventures in the Caribbean. It was the South that was the strongest proponent of invading other countries and seizing their land, mostly to expand slavery to new southern environs.
So to picture the North as the imperialists is to ignore the decidely imperial goals of the Southern Confederacy.
The revisionist attack on Northern goals is therefore doubly wrong.
And yes-- as I noted, this historical slander on the Radical Republicans had purposes in the past, namely to justify defense of Southern Democratic racists, just as your arguments are used by analogy to justify leaving an oppressive dictator like Milosevic in occupation of Kosovo.
The ideological need to undermine the idealistic nature of both the Civil War and World War II are tropes of a range of isolationist leftism, which is as morally blind as Bush's ideological unilateralism. In a sense, they are related in their decided view that there are no international standards that should either restrain or compel action-- merely unilateral imperatives of action (or inaction) for the US. That Milosevic or Bush might violate international law and decency are both ideologically irrelevant in each case.
-- Nathan Newman