Here is a concise thumbnail description of Lee's gamble in the North: the Battle of Antietam:
"Lee repeatedly broke and drove back the advancing Union armies. Then in the summer of 1862, he took the aggressive and invaded the North. His eager and victorious soldiers hoped to sweep successfully over the entire country. But they were met in Maryland at Antietam Creek by the Union army commanded by General George McClellan. The battle that ensued was the bloodiest and the most costly single day of strife in all this awful war."
More about the American Civil War can be read here
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