By chance, McClellan had acquired a set of Lee's orders, revealing that his divisions were widely separated, and could be destroyed piecemeal.
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The offensive was launched by the South, not the North. The particular advantage enjoyed by the North was a set of Lee's orders that a Confederate officer had lost in the field, and were shown to McClellan. These orders revealed that Lee's divisions were widely separated, and could be destroyed piecemeal. McClellan did not move fast enough to exploit this advantage to the full, but Lee had to concentrate his forces in a hurry and fight in a location that was unfavourable to him.
The primary advantage for the South in its 1862 offensive into Maryland was the demoralized state of many of the Northern troops in the opposing army, as well as the inexperience of the new recruits who had just been called up in order to make good on recent Union losses. Additionally, the outstanding leadership of General Robert E. Lee remained a key Southern strength.
It was the South that had the advantages - a superior General and an army in high morale - until a Confederate officer accidentally dropped a set of Lee's orders in the field, where they were found and shown to McClellan, who was then able to exploit the wide gaps in the Confederate lines. Only the presence of a Confederate spy in his camp (able to alert Lee) prevented him from destroying the Army of Northern Virginia.
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