Shortly after his inauguration in March 1861, Abraham Lincoln made General Ervin McDowell the commander of the Union Army.
On July 25, 1861, just four days after the Union defeat at Bull Run, Lincoln relieves McDowell of his leadership and places General George McClellan as First Commander of the Union forces.
On July 27, 1861, McClellan takes command of the Army of the Potomac.
On November 1, 1861, Lincoln relieves General Winfield Scott as General-in-Chief of the US Army and replaces him with McClellan.
On March 11, 1862, Lincoln relieves McClellan from his position as General-in-Chief of the Federal Armies.
On this day, July 23, 1862, Lincoln makes General Henry W. Halleck General-in-Chief of the entire Union Army and tasked him to unify and coordinate the Union forces, which had been less than successful thus far in the war against the Confederacy.
Henry Wager Halleck was born in 1815 in New York. In 1839, he graduated third in his class at West Point. Halleck was considered such a bright cadet that he had the rare honor of being asked to teach while still a cadet. He started with the Army Corp of Engineers and helped to fortify the defenses in New York harbor. He served during the Mexican-American War. Between the Mexican-American war and the outbreak of the Civil War, Halleck had managed to become the president of a railroad as well as get involved in California politics. When the Civil War broke out, he received a Union commission making him a major General. He was assigned to Missouri where he is credited with organizing the Union forces there and preserving Missouri as part of the Union. Although Halleck was a brilliant military student and officer, he often quarreled with Union leadership, including Union commanding Generals Burnside, Hooker, McClellan and Meade.
On September 2, 1862, Lincoln restores McClellan to command of the Federal Division of the Potomac.
On November 5, 1862, Lincoln again relieves McClellan and gives command of the Army of the Potomac to General Ambrose Burnside.
On January 25, 1863, Burnside relieved of command of the Army of the Potomac. General Joseph Hooker given command of the Army of the Potomac.
On June 28, 1863, Lincoln relieves Hooker of his command of the Army of the Potomac and gives the command to General George Meade.
On October 16, 1863, General Ulysses S. Grant given command of the Union Military Division of the Mississippi.
On March 9, 1864, Lincoln places Grant in charge of all of the Union armies. He appoints General William T. Sherman to replace Grant as commander of the Mississippi Division, also called the Army of the West. At the same time, Halleck is made Chief-of-Staff for all the Army. In this position, he organized and supplied Grant with the supplies he needed to defeat Lee and the Confederate Army. Halleck obtained the nickname ‘Old Brains’ due to his knack for his military and bureaucratic talents. It was clear to Lincoln and others that Halleck was a far better military organizer than a military strategist.
On April 9 1865, Confederate General Robert E. Lee surrenders to Union General Grant at the Appomattox Court House in Virginia.
After the Civil War ended, Halleck remained in the Army and was assigned to command the Military Division of the Pacific out of San Francisco. In 1869, Halleck was transferred to Louisville, Kentucky where he was assigned to the Division of the south. Halleck died in 1872, while still actively serving the Army and is now buried in Green-Wood Cemetery, Brooklyn, New York.
Sources for the above includes: Today, July 21, 1861: First Battle of Bull Run; 150th Anniversary Timeline: 1861-1865; The U.S. Civil War 1861-1865; Halleck Takes Command of the Union Army; Civil War Timeline; Henry W. Halleck; American Civil War: Major General Henry Halleck