Who was the author of the mystery document?

But now The Papers of Abraham Lincoln, through research by associate editor Stacy Pratt McDermott, has found that the letter was written by Andrew Johnston, a newspaper editor, lawyer and fan of Lincoln's poetry

Explanation:

one of the main reasons was definitely slavery. back in 1861, republican abraham lincoln was elected and ended up being a major influence in the slavery business. slavery greatly affected the texas economy because back in 1845 during the annexation of texas, slaves became a big part of the texas population. many texans that were in favor of the secession influenced groups through party ties.