The "north" wasn't the ones who started shooting. It was the secessionists that chose war.The North figured that Southern Independence would cost them more than war - so they opted for war.
"In the North the effects of the war, we are told, were everywhere visible. In the great commercial metropolis of New York, business was almost entirely suspended, and many of the largest mercantile houses had been forced to close. Every department of trade was paralyzed, and the streets even told of the depression of business. 'For sale' and 'To let' met the eye at every step, or some other evidence of the fast failing fortune of the North..." -Daily True Delta (New Orleans, LA), April 11, 1862