In the eyes of Simon, what made the Lincoln and Grant relationship successful was they had a mutual trust and respect for each other. Lincoln trusted Grant in everything he did, sort like how Robert E. Lee trusted Stonewall Jackson in all of his campaigns. With previous generals Lincoln had to balance both political and military aspects of the war because he had no trust with the other generals, but with Grant he could primarily focus on the political aspects of the war because he trusted Grant wit h the military aspects of the war and remaining aggressive throughout it.
Grant was known for being an aggressive general and not being afraid to take risks with his army, and would also look to achieve the devestating strike to defeat the enemy army. Even if he was losing a battle, he was able to strategize how to counter attack and go back on the offensive. One of Grants biggest strengths was that he was able to learn his mistakes and adapt his strategies like seen at the Battle of Shiloh and at the Battle of Vicksburg. and how he lead the Union army. Compared to McClellan who always believe his ideas were correct and never wavered them, Grant became a huge part of the success of the Union army.
Lincoln’s relationship with Grant was the polar opposite of Lincoln’s relationship with McClellan. Although McClellan could rally and train his men well, outside of that Lincoln had no trust for him as commander of the Union army and McClellan a lot of the times refused to listen to Lincoln, one time forcing Lincoln to go to tell McClellan to attack, but he claimed the horses were to tired to attack. There was just no trust or even a high level of respect between the two. Grant and Lincoln on the other hand had blind trust for each other and could rely on each other as well for advice and remained strong with his aggressiveness. Lincoln became more effective as a wartime president because he saw more victories with Grant leading the Union army.
The Union command eventually developed the capacity to win the war because under Grant the entire army became a cohesive unit fighting for the same goal and no longer had to deal with incompetent generals. At the start of the war the Union high military command struggled in finding its leader, but when it finally did that’s when it ultimately found the most success, which was the opposite situation that occurred to the confederacy with strong generals at first, but then losing leadership as the war went on.