Emory Upton went through the Civil War as a McClellan hater; after the war, with study and reflection, he became an admirer. Being brother-in-law to Frank Blair might have softened him up.
Upton was a prodigy, entering the Civil War as a second lieutenant of artillery. By the end of the war, at 25, he was a general who had led all three branches well. He finished as a division commander. Sixteen years later, he committed suicide.
After the war, Sherman sent Upton to Europe as a one-man Delafield Commission. He returned with recommendations, famous among which was that the U.S. should model the Prussian general staff. He is viewed now as the father of the American general staff.
McClellan and the Delafield Commission had missed that recommendation. McClellan had friends on the Prussian General Staff before the war. After the war, McClellan knew von Moltke the elder - himself! - (who complimented him on his first Richmond campaign) and GBM knew the pre-Franco-Prussian War Prussian General Staff; likewise the Prussian General Staff knew McClellan and read reports from one of their own that Grant's campaign of 1864 had been but the restart of McClellan's early 1862 strategy.
But McClellan and the rest of the Commission had missed an opportunity to advise Jefferson Davis to start a Prussian General Staff in America and Sears is very hard on McClellan in particular for that failure. Sears has many followers in this.
The foundation of an Army War College is another of Upton's ideas and he is justly credited for it today. (The son of the political Civil War Gen. John M. Palmer would attend the Army War College founded on Upton's recommendation.)
We could list many realized ideas advocated by Upton but the biggest of them all is the eternal notion of the expansible army. This originates with Calhoun but Upton made it his own, modernizing it in the form implemented in the 20th Century. The expansible army consists of a large regular standing army, men and officers, with a capability of expanding further in crisis. Upton rejected the idea of a small standing army bolstered in war by militias and volunteers. Upton proposed to learn from America's wars, especiall the ACW.
The Army as we knew it in recent times, through the first Gulf War, was Emory Upton's child. Upton's ideas are doctrine - even gospel - today.
At the time of his suicide (1881), Upton the theorist, was a failure. His reputation grew after his death and the first major effort to legally establish an expansible army came after WWI.