Historian James McPherson is justly famous for having written Battle Cry of Freedom: The Civil War Era (Oxford History of the United States) -not only the best single volume book of the US Civil War, but one of the very best books of History on any country at any time. Battle Cry of Freedom offered a unique synthesis of Civil War scholarship in a brilliantly written and meticulously considered book. Since "Battle Cry of Freedom", McPherson has struggled in vain to produce anything as remarkable. His output has varied from reflections on the importance of the Civil War and Abraham Lincoln (Abraham Lincoln and the Second American Revolution), to interesting if predictable research into the motives of the Civil Warriors (For Cause and Comrades: Why Men Fought in the Civil War), and even to fairly forgettable accounts of major battlefields (Hallowed Ground: A Walk at Gettysburg (Crown Journeys)).
In "Tried by War" Lincoln tackles a subject he has highlighted earlier as a gap in Civil War Scholarship: The role of Abraham Lincoln as the Supreme Commander of the Union, and his contribution to the military victory.
Under the surface of "Tried by War" hides a fascinating, and to the best of my knowledge untold, story of the political and institutional changes of America. While McPherson at times senses that something is missing his narrative, reflecting on the political reasoning behind the military decisions, for the most part he settles with repeating - sometimes almost verbatim - the main narrative from "Battle Cry of Freedom". That this is nonetheless a readable, entertaining, and at times enlightening read is a great tribute to McPherson's capacities as a storyteller and a - conventional - historian.
McPherson's narrative focuses on two main aspects of Lincoln's contribution: His attempt to maintain public support for the War, and his struggles to make his Generals execute his military strategy rather than their own. The rest of my review would focus on the latter issue exclusively.
While Historians generally try to explain historical events by analyzing the interactions of complex historical trends and forces, Military history seems to have remained content with describing all history in terms of personalities. "Tried by War" definitely sins in this regard - in the main, it is a description of Lincoln discarding unsuitable Generals (George B. McClellan, Winfield Scott, Joseph Hooker, etc), and promoting the right ones (Chiefly Grant, but also Sheridan, Sherman and Thomas).
McPherson points out that Lincoln had a strategic vision of the war which differed greatly from that of his Generals, particularly General McClellan. This alone was not remarkable; Political leaders often have very different views from those of their Military advisors. What was remarkable about Lincoln's strategic insights were that they were obviously superior to those of the military, and that Lincoln managed to get the army to execute them. These accomplishments are all the more remarkable given Lincoln's lack of either military or high-level political experience.
Lincoln's proposed Grand Strategy for the War involved the destruction of Confederate Armies; It called upon the Union to take advantage of its superiority of numbers to conduct major military operations simultaneously - what McPherson calls "concentration in time" so as to prevent the Confederates from shifting resources each time against one Union army or the other. The strategy of Lincoln's early Generals was a strategy of a War of movement and encirclement (e.g. Winfield Scott's "Anaconda Plan"), designed to subdue the rebellion without major set battles.
As I said, other leaders have disputed the strategies suggested by their generals. But even with hindsight, it is not easy to say who was correct. Hitler's military adventurism seemed deranged to his senior commanders, but was it? After all, Hitler's bold action had paid handsome dividends, as with the invasion of Austria, Czechoslovakia, Poland, and of course France (Fateful Choices: Ten Decisions That Changed the World, 1940-1941). British Premier Lloyd George wanted the military to change the focus from the deadly Western Front to a sort of military action in the East. While the popular view of the Great War is one of "Lions led by Donkeys", many military historians agree with the military (See Mud, Blood and Poppycock: This Will Overturn Everything You Thought You Knew about Britain and The First World War (Cassell Military Paperbacks) and Forgotten Victory: The First World War - Myths and Realities (Systems and Control: Foundations and Applications)). Even Jefferson Davis had envisioned a military strategy different than the one executed by the Confederacy - one modeled on George Washington's policy of preserving the armed forces and avoiding battle except under positive circumstances (But Confederate strategy was shaped above all by Robert E. Lee - and how many people are willing to consider that Lee erred? - Lee Considered: General Robert E. Lee and Civil War History).
In my opinion, the source of Lincoln's superior military insight comes from political, rather than personal, grounds. The Pre Civil War army was a predominantly Southern, and especially Virginian, institute. The last three Secretaries of War were all Southerners; Its commander, Winfield Scott, a Virginian (albeit a loyal one). Many of its highest and most admired officers were Southerners as well (Primus inter pares, of course, was Robert E. Lee).
While I have no information on the Northern officers, I imagine most of them were sympathetic to their Southern colleagues, and thus mostly belonging to the Democratic party (McClellan, Rosecrans, Burnside were all Democrats, although Winfield Scott had been a Whig). The Democratic Party was a political coalition, where the senior partners were the Southern elite. The kind of War Lincoln and the Republicans called for would mean massive destruction of Southern lives and treasure, undermining the power of the Southern Democrats and solidifying Republican control. No wonder Democratic Generals disapproved. This was possibly a source of difference between Lincoln and the Generals: War as the continuation of Party Politics through other means.
Also remarkable was Lincoln ability to discard Generals who failed to do his bidding, and to promote those who did. Lloyd George loathed Field Marshall Haig, but was unable to oust him (See Lloyd George: War Leader, 1916-1918 (Penguin Biography)). Truman has had great difficulties in retiring General McArthur, and George W. Bush has had to await an electoral defeat before reshaping the Iraqi military strategy around "the Surge".
This is another question McPherson does not discuss. I would speculate that the answer is institutional. Unlike the armed forces in other conflicts, the US Army at the war's outbreak was in a state of chaos. Many of its best officers - including the designated commander -defected, and went to fight for the other side. Thus, deprived of key personnel, the army had to expand to numbers approximately 50 times its pre war size, and to prepare for a war more savage than anything it has known. Other expansions of the armed forces relied upon a skeleton of military professional. The army of the Civil War was commanded by soldiers who returned to active service after lucrative careers outside of it (unlike the rebuilding of US Forces in the First and Second World Wars, which were commanded by career officers such as John Pershing and George Marshall). If the President was "green", he faced a military establishment less established, so to speak, than other leaders.
None of this is to disparage Lincoln's accomplishment, but only to try and set them in perspective. McPherson doesn't attempt to do this, and thus fails to really tell us much new about the 16th president's generalship.
"Tried for War" is as well written as one can expect from McPherson (although, unlike his previous books, it contains excessive melodrama: "Has it not been for Lincoln's support at this time, the Grant of history would not have existed - and perhaps neither the Lincoln of history" p. 85). If you take it for what it is - a narrative of Lincoln as commander in chief - it is both adequate and entertaining.