Who military leader of the union army?

There were several leaders:

Winfield Scott: July 5, 1841 -- November 1, 1861

George B. McClellan: November 1, 1861 -- March 11, 1862

Henry W. Halleck: July 23, 1862 -- March 9, 1864

Ulysses S. Grant: March 9, 1864 -- March 4, 1869

Abraham Lincoln: between McClellan and Halleck