At the time of Lincoln’s second inaugural address, the US the Union’s victory over the South was imminent. Because of that Lincoln delivered a brief speech that explained the reason for the Civil War - slavery and the use of slaves -. The resolution of his speech was to recover from the war and do not let this happen again.
The second speech was different than the first because at the first speech people thoughts and attention was to start a war to secede from the Union and the second speech was about union and recovery from the Union.