What was Lincoln saying?

A letter-to-the-editor published in the Fredericksburg newspaper quotes a suggestive and cryptic statement from Abraham Lincoln:
You and I both anticipated that the cause of the country would be advanced by making the attempt to provision Fort Sumter, even if it should fail; and it is no small consolation now to feel that our anticipation is justified by the result."--Lincoln, in a letter to Gustavus Fox on May 1, 1861.
To the letter writer, this is the "smoking gun" that proves Lincoln sought war. Is that what it means?

The writer quotes Karl Marx:
The war between the North and the South is a tariff war. The war is further not for any principle, does not touch the question of slavery, and in fact turns on the Northern lust for sovereignty.
If you look at it from the outside, for instance if you consider cotton revenues as a proportion of the national income, this is plausible. The U.S. then was like one of the modern oil sheikhdoms. The counterpart to today's "No blood for oil" would naturally have been, "No blood for cotton."

But these economic arguments are reductionist approaches to understanding complex historical events; they look for the single driver. They are the flip side of the incomplete argument "slavery caused the war."

What baffles is why Lincoln (a supposedly sophisticated thinker) would flirt with reductionism - "our anticipation is justified by the result."