Final answer:
Lincoln fired McClellan due to his cautious military approach and failure to capitalize on opportunities, seeking a more decisive leader to hasten a Union victory.
Step-by-step explanation:
President Abraham Lincoln's decision to fire General George B. McClellan was based on a culmination of events showcasing McClellan's hesitancy in battle and ineffective leadership. McClellan's conservative approach, particularly during the Battle of Antietam on September 17, 1862, and his failure to pursue General Lee aggressively, led to Lincoln's frustration and the eventual replacement with General Ambrose E. Burnside. It's clear that Lincoln sought a commander who would take decisive action, aimed at securing a Union victory and potentially shortening the war. While the removal of McClellan was not solely responsible for the outcome of the war, it was reflective of Lincoln's determination to find leaders capable of taking decisive actions, thereby affecting the war's duration and eventual conclusion.