Re: Lincoln's Letter to the Editor of the N,Y, Tribune

This really isn't difficult. I fail to see how you have so many problems with it.
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Yes, Lincoln's goal was to preserve the Union.
Why did the Union need preserving?
Because slave states seceded. (Not "succeeded".)
Why did those states secede?
Slavery.
Don't take my word for it.
Look up the Articles of Secession. Read the Cornerstone speech.
It's all written down.
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You are right. This was the South's reason for the secession, but secession was due to a misunderstanding of Lincoln's objective. This based upon Lincoln's expressed words as to his objective in his letter almost 2 years after the start of the war.