Lincoln and Grant’s partnership was very powerful because they played off of each others’ strengths and weaknesses so well. Grant handled the military well enough for Lincoln to spend more time in political arenas and push agendas to help the Union Army win the war.
Grant was a typically aggressive commander, and spent much of his time on the offensive. Grant’s dominance on the Western side of the war was a tremendous help to the United States.
Grant and McClellan are easy to compare, in the sense that they are polar opposites not just as commanders but in terms of their relationship with Lincoln. Grant played well with Lincoln and was aggressive and effective, whereas McClellan often argued with Lincoln and was notorious for his overly-cautious tactics on the battlefield.
Lincoln’s command tactics found success because he found good leadership to delegate tasks and responsibilities to, so he wouldn’t be constantly be burdened and be able to focus on important matters at full mental capacity.