Every workplace has them: the loudest, most boisterous employees, constantly talking about how much work they're doing and how good they are at their jobs or making a scene with their after-work activities. Meanwhile, quietly plugging away somewhere, there are the employees who really are good at their job, their performance going unnoticed because they simply just want to finish up and go home.
The Union Army in the Civil War was no different. Grant struggled with alcohol, Sherman had to work to maintain his sanity, and George B. McClellan just knew everyone in all the right places. Meanwhile, these guys were chugging along, slowly winning the Civil War.
Samuel R. Curtis
Missouri is likely a forgotten theater of the Civil War, but for the Union at the outset of the war, Missouri was the one bright spot that shined through an otherwise dreary day. The reason for that is Samuel R. Curtis. While the Union Army in Virginia was spinning its wheels, Curtis was kicking the Confederate Army out of Missouri and into Arkansas. For the rest of the war, he would be bogged down in insurgent violence in the region (Kansas was a violent mess before the war even started).
The Civil War West of the Mississippi was dominated by the Union Army, and it's largely because of Curtis.
You may not have heard of Nathan Kimball, but that's okay because he has one thing most Union generals could never have: a victory over Thomas J. "Stonewall" Jackson – in the Shenandoah Valley, no less. Kimball was a doctor and veteran of the Mexican-American War who assumed command of the 14th Indiana at the start of the Civil War. As Jackson began his famous 1862 Shenandoah Valley campaign, he tried to knock out a force a Kernstown that was guarded by the 14th, but it was the Hoosiers there who gave Jackson the bloody nose instead.
Kimball's unit then went on to earn the nickname "The Gibraltar Brigade" for their assaults on the sunken road at Antietam. His future victories came at places like Fredericksburg, Vicksburg, and he was a division commander during the Battles of Franklin and Nashville that destroyed the Confederate Army in Tennessee.
The Civil War broke out after Prussian general August Willich emigrated to the United States. Never one to bow away from a fight, he decided he would stand up and defend his adopted homeland by raising a unit of German immigrants and drilling them into a crack Prussian unit the likes of which the Confederates had never seen.
Despite being briefly captured and held prisoner, Willich's Prussians performed like champions at Shiloh and Chickamauga but it was his unit that broke the Confederates at Chattanooga.
Gen. George H. Thomas
Thomas might be the most underrated General of the entire Civil War. In January 1862, Thomas was leading a simple training command in Kentucky but Confederate movements forced him into a fight. At the Battle of Mill Springs, it was George H. Thomas that gave the Union its first significant win of the war. Thomas would go on to finish the war undefeated but unglorified – because he moved slowly and deliberately, caring more about his men than about his legacy as a commander.
He was responsible for some of the most key Union wins of the war. His defense at Chickamauga saved the Union Army from destruction and his later victory at Nashville completely destroyed the Confederate Army of Tennessee under John Bell Hood.