Final answer:
Lee's greatest victory against the Union army came in 1863 at the Battle of Gettysburg.
Step-by-step explanation:
Lee's greatest victory against the Union army came in 1863 at the Battle of Gettysburg. It was a turning point in the American Civil War and a significant defeat for Lee's Confederacy. The battle lasted for three days and resulted in heavy casualties on both sides, but ultimately the Union forces were able to repel Lee's army and secure a crucial victory.
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