It was after the Battle of Antietam. McClellan had a perfectly good opportunity to pursue and destroy Lee's army while it was in a vulnerable position, but he moved far too slowly, and Lee was able to get his army back to Virginia.
General McClellan blamed President Lincoln for the Union disaster in the Peninsula campaign. On June 28, 1862, as he begins his retreat back to Harrison's Landing, he sent a telegraph to Lincoln, accusing him and Secretary of War Stanton of sabotaging his campaign. Lincoln returns the message that he will support McClellan, ignoring McClellan's hostility. Realizing the unstable military position of the North, Lincoln began to call for 300,000 new recruits from the Northern state governors.
McClellan was relieved of his command on October 7, 1862, by President Lincoln for a number of reasons, including being insubordinate to the President, fighting with General Winfield Scott, and for being much too cautious with the enemy, often failing to press forward to win a battle.
Because he had caused so many delays tha some of Lincoln's cabinet were starting to doubt his loyalty to the cause, and after the Battle of Antietam, he failed to pursue and destroy the enemy.
The reasons behind Lincoln's dismissal of McClellan were complex. With that said, President Lincoln had few choices. Ambrose Burnside replaced General George B. McClellan after the Battle of Antietam. Prior to his final dismissal in November of 1862, McClellan was stripped of his army in favor of the new army under General John Pope. Most people believed that McClellan's career was over. However, Pope's defeat at the second Battle of Bull Run saw Lincoln restore McClellan as the leader of the Army of the Potomac. Lincoln's cabinet objected to give McClellan command of the Army of the Potomac, but Lincoln correctly reasoned that this army loved McClellan and would fight their best under his command.
His own natural caution, compounded by some vastly exaggerated estimates of enemy numbers, supplied to him by Pinkerton.
yes
He wasn't. Meade wasn't replaced at all. He was in at the end. McClellan had been replaced by Burnside after the Battle of Antietam in September 1862 for failing to pursue and destroy Lee's army.
More than likely your alternate is failing.
Because his health was failing.
Halleck himself was never in a position to attack Lee. When he was appointed General-in-Chief in July 1862, he summoned McClellan back to Washington and ordered him to attack Lee in the Shenandoah. Lee defeated part of the Union army before McClellan was able to reach the spot. Next came the unexpected Union win at Antietam, but Halleck and the whole of Lincoln's cabinet criticised McClellan for failing to pursue and destroy Lee's army, and McClellan was replaced. Halleck then supervised the two further failed attacks on Lee (Fredericksburg and Chancellorsville), and the successful one at Gettysburg. The next campaign involving Lee was not till May 1864, by which time Halleck had been replaced.
Antietam (or Sharpsburg) is considered a turning point of the American Civil War for several reasons. The battle brought to an end the first Confederate campaign north of the Potomac River, whether one considers the movement that brought the Confederate army there an invasion, or a big raid. Another result was the battle and what followed gave Lincoln the excuse to at long last rid himself of General George McClellan. McClellan, despite outnumbering Lee three to one, failed to destroy Lee's army, when Lee was fighting with his back to the River, which was in flood stage, making escape for any part of Lee's army forced to flee from a defeat all but impossible. As Lincoln, untrained in military matters but a very quick learner, said: "He had them in the palm of his hand, and had only to close his hand about them". McClellan failed to get large portions of his army into the battle; two of his seven army corps never fired a shot all day. McClellan basically fought three separate battles, on the north end, the middle, and then the south end of the field, failing to make these efforts simultaneously, allowing Lee to move forces around to meet each individual threat. Thus Lee was able to hang on, though it was a very, very near thing several times. Lee knew his opponent, and the day after the battle remained on the field, daring McClellan to try again, and then, the River having gone down some, was allowed to depart over the River (and a River crossing in the presence of the enemy is extremely dangerous, if that enemy attacks while the army is in the midst of crossing). McClellan then crowed about having "driven" Lee out of Maryland, failing to grasp that he had left intact the Rebel army, to fight again on many more fields. McClellan truly had the chance to end the war that day, and seems not to have understood that whatsoever, so it would take two and one half more years to bring the slaughter to an end. But McClellan was an important Democrat, and Lincoln had to handle him carefully. Weeks after the battle Lincoln visited McClellan and his army, still on the field at Antietam, with no idea where Lee was south of the River, in an effort to nudge McClellan into motion, and get him going after the enemy. But McClellan could not be moved. Lincoln still had to wait until after the midterm elections in early November, 1862, but within a couple of days after those elections, McClellan was relieved of his command, and was never given another command during the rest of the war. Of the half dozen commanders of the Army of the Potomac Lincoln tried before finding Meade and Grant, McClellan lasted the longest, but finally getting rid of him started the long and painful search process, involving many failures, which finally allowed Lincoln to obtain a more competent commander for his main field army. But the most important result of the battle, and perhaps the biggest reason the battle was a pivotal turning point, was the effect overseas. Britain and France had been on the verge of intervening in the war, which would have meant the Confederacy would have achieved its aim, of establishing itself as an independent nation, and so would have basically won the war. Lee had given as one of the reasons for making this move into the north that he hoped "to conquer a peace" there, by winning a big battle on northern soil, thus encouraging Britain and France to intervene on his side. Failing to win, though he did not lose either, this did not happen, and ended the closest flirtation Britain and France had with the idea of intervention. After the battle Lincoln issued the Preliminary Emancipation Proclamation (he had been waiting, for months, for a Union victory to do so), and after that, any foreign intervention would have seemed to be an intervention in favor of slavery. Neither Britain nor France were willing to be seen in that light.
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