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 McClellan had been replaced by Burnside after the Battle of Antietam in September 1862 for failing to pursue and destroy Lee's army.
The commander of the Unions Army of the Potomac at Antietam was Major General George B. McClellan.
First, the Battle of Bull Run in July 1861 - a premature attempt to march on the Confederate capital, when neither side was ready for operations. Second, the Peninsula campaign, April 1862 - McClellan threatening Richmond with a big army arriving by sea. McClellan turned out to be all talk, no action. Third, Halleck's plan for McClellan and Pope to join forces, and overwhelm Lee through sheer numbers (August 1862). The unbeatable Lee-Jackson partnership saw them off. Fourth, the newly-appointed Burnside trying to dislodge the Confederates from the high ground at Fredersicksburg (December 1862). A good plan that suffered from delays, to which Burnside was not agile enough to react. Fifth, another attempt by the same army, now under Hooker, to destroy Lee, resulting in a devastating defeat at Chancellorsville.
George B. McClellan & Robert E. Lee
General Lee defeated Pope in the Second Battle of Bull Run.
Union General George B. McClellan received the battle plans for the Confederate invasion of Maryland on September 13, 1862. He recognized the value of knowing Lee's plans and the opportunity he had to catch and defeat Lee. McClellan immediately informed President Lincoln of this opportunity.
Robert E. Lee
On September 16, 1862, one day before the historic Battle of Antietam, Major General George B. McClellan lost his best opportunity to defeat General Lee's army in detail in Maryland. McClellan was slow to assemble his forces in and around the town of Sharpsburg, Maryland. McClellan was unaware that he substantially outnumbers Lee's army and postpones an attack on the Confederates. Instead he waits another day until his reserve forces joins his main army. Theoretically, if McClellan conducted an assault on September 16, the Battle of Antietam would have been a smashing Union victory.
The 1862, the Battle of Antietam was fought in Maryland. The Army of Northern Virginia fought against the Army of the Potomac. It was Lee versus McClellan.
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 McClellan had been replaced by Burnside after the Battle of Antietam in September 1862 for failing to pursue and destroy Lee's army.
McClellan forced Lee to retreat at Antietam.
On July 1, 1862, the Battle of Malvern Hill was fought in the Peninsula campaign. Confederate forces under General Robert E. Lee suffered a serious defeat against entrench Union troops. Union commanders under General McClellan urge a counterattack, however McClellan believes he needs to retreat rather than lose more troops.
The Battle of Antietam took place on September 17, 1862. At the onset of the battle most of Lee's troops were in defensive positions to the west of the Antietam Creek. The town of Sharpsburg lay between to sections of Lee's army. Lee had made his headquarters on the southwest side of the town of Sharpsburg. Union General George B. McClellan was headquartered to the northeast of Lee. McClellan had made the east side of Antietam Creek. He was directly next to the Creek itself.
Battle of Antietam/Sharpsburg Maryland. The Union would have defeat General Lee, but, General McClellan was a wimp and President Lincoln replaced him afterwards. The Emancipation Proclamation was announced after the battle by President Lincoln, becoming effective on Jan. 1, 1863.
Summer of 1862
The Battle of Antietam, an important battle of the Civiil War, was fought by the Army of the Potomac, part of the Union Army, under the command of General George McClellan, and Confederate forces under the command of General Robert E. Lee on September 17, 1862.
On September 15, 1862, Major General George B. McClellan made a miscalculation that would have made a significant impact on the Battle of Antietam. In fact, that battle would have taken place on that date instead of September 17, 1862. On the 15th McClellan could have attacked General Lee's forces at Sharpsburg but he hesitated. He believed he did not have enough troops, when in fact he had 40,000 troops as compared to Lee's 18,000 troops. Clearly, this would have changed the history of the US Civil War, and may have ended that war in a very short time, as Lee's army would have been devastated.