The newly inaugurated sixteenth President of the United States, Abraham Lincoln, was deferential to the judgment of his generals throughout the first year or so of the Civil War but eventually realized that he must take a much more active role and eventually became the first president who was also (in effect) the nation's wartime commander in chief. First, he replaced Winfield Scott with George B. McClelland, then McClelland with Henry W. Halleck, then Halleck with George G. Meade, and Meade with Ulysses S. Grant. Only with Grant was Lincoln finally confident that he had in his last general-in-chief the combination of strengths (e.g. judgment, will, focus, and tenacity) that were sufficient to the challenge of winning the war and thus preserving the union. However, even then, he still played a dominant role until Lee's surrender at Appomattox Courthouse on April 9, 1865. Six days later, Lincoln was dead.
There were warrior-monarchs throughout history who led their armies to great victories. However, they received much better preparation to do so than did Lincoln. Technically, he led no troops into battle as had Odysseus, Alexander, Genghis Khan, and Henry V but he probably spent more time with his generals and among their troops than has any U.S. president since then. It should also be noted that, since childhood, Lincoln had an insatiable desire to learn and the determination to obtain and the self-discipline to apply effectively what he learned about law, the political process, and eventually waging war. The title James M. McPherson selected for his book is especially appropriate. As he notes, Lincoln "was not a quick study but a thorough one" and over time, through rigorous study, learned everything he could about relevant military history, especially military strategy and tactics. Several generals (notably McClelland and Don Carlos Buell) suffered from what Lincoln characterized as "the slows" and most others were risk-adverse. Time and again, Lincoln reminded them that their #1 priority was to destroy the Confederate army. Only Grant "got it" and the men he promoted (i.e. William Tecumseh Sherman, Philip H. Sheridan, and George H. Thomas) shared his determination to force an unconditional surrender as soon as possible.
To be accurate, all of the Union and Confederate leaders were "tried by war." McPherson carefully explains how and why some emerged much stronger from their exposure to severe and prolonged pressures during the war and others did not. In this context, the metaphor of a crucible seems apt. That is, the development of leadership skills resembles somewhat the process by which alchemists attempted to transform base materials into precious metals by subjecting them to intense heat. There are those who believe that an extended ordeal such as the Civil War develops character. McPherson agrees, asserting that no other person's qualities of character proved more important to the preservation of the Union than did Lincoln's. Precious metal indeed.