Lincoln was remarkably consistent in his idea that the nation cannot continue on a path where it represents "half free" and "half slave."  He did not want to be the aggressor and took steps to bring the South into the fold. His Inauguration Speech in 1861 demonstrated the ends to which his beliefs about maintaining the Union were possible:

We are not enemies, but friends. We must not be enemies...The mystic chords of memory, stretching from every battlefield, and patriot grave, to every living heart and hearthstone, all over this broad land, will yet swell the chorus of the Union, when again touched, as surely they will be, by the better angels of our nature.

The "better angels of our nature" is a direct reference to seek to keep the Union together and this preservation is something that Lincoln followed for as long as possible. While Lincoln did believe in war as a last option, he did not relent when Southerners wished to secede, suggesting that such action was not going to cause him to reevaluate his positions and somehow acquiesce.  This brings out the idea that one way or another, America would come to a resolution regarding its status as "half free" and "half slave."    Lincoln recognizes clearly that war should be the last option, reflective in how much he avoided the issue in terms of not wanting to be the first to fire on the South.  However, with the attack on Fort Sumter, Lincoln did not surrender, but rather intensified his efforts to demonstrate his willingness to meet the South directly in confrontation.  It is to this end that Lincoln's beliefs are clearly demonstrated in how the nation could not exist as one that was divided.  Lincoln's call on states for troops after Fort Sumter becomes the clear bell ringing that shows his belief as America not remaining as a "house divided."