Robert E. Lee, a hero in the War with Mexico, did not accept command of the Union Armies during the Civil War, because he felt he owed more loyalty to the state of Virginia than to the country of the United States of America.
Burnside led the Union troops.
Lee did not accept command of the Federal forces because he felt he owed his allegiance to his state, Virginia, rather than to the whole country. He probably did expect Scott to offer him the post.
Robert E. Lee
Commanding the Union Army meant that Lee would have to fight against Virginia, which was his home state. Lee refused to betray his home even though he didn't believe in slavery.
Robert Edward Lee before he resigned his commission in the U.S. Army was offered Command of all Union troops. He declined,resigned and offered his service to his home State of Virginia.
The Union's military high command was aware that Confederate troops were forming a large concentration for what would be the Battle of Chickamauga. They attempted a counter concentration that called for Union troops in Georgia and as faraway as Minnesota. None of these troops were able to reach Chickamauga until after the battle was over.
George b McClellan was a Union general. All the generals do is command their troops.
With Grant now in command of all union troops and tough Union commanders in charge of divisions at Chattanooga, Union troops overcame strong Confederate resistance. They attacked a superior Confederate stronghold at Missionary Ridge, turned the tide, and gave the Union a brilliant victory.
Robert E. Lee
He called for additional troops and put general George B. McClellan in command
Under the command of Union General James Harrison Wilson, his army of 9,000 troops were successful in defeating the forces of Confederate General Nathan Forest. Forest had a defending army of 4,000 troops. Confederate forces took on heavy casualties in the battle.