Final answer:
Robert E. Lee marched his Confederate army to Gettysburg to secure a strategic victory for the Confederacy and potentially weaken the Union's resolve to fight.
Step-by-step explanation:
Robert E. Lee's reason for marching his Confederate army to Gettysburg was to secure a strategic victory for the Confederacy. Lee wanted to bring the war to the enemy and potentially end the war sooner. He hoped that a significant Confederate victory in the North would weaken the Union's resolve to fight and potentially gain support from Great Britain and France.