This is not no secret. I learned this in 4th grade history class.

In the statement from your posted letter:
[This is the beginning of the war which every statesman and soldier has foreseen since the passage of the South Carolina ordinance of secession.]

The debate which can be concluded here seems to be therefore, was it the acts of Lincoln sending troops to South Carolina, or was it the acts of South Carolina's ordinance of secession?

Did South Carolina violate any Laws of that day that justified Lincoln to send troops, or did Lincoln violate the Law in sending troops? Or were both in comliant with the law of the day?