What was the main reason that the Federals lost control of Gettysburg on July 1st 1863 during the Battle of Gettysburg?

Answer 1

Battle of Gettysburg (July 1-3, 1863)

The primary cause for the Union failure on the first day of Gettysburg was the hurried deposition of the Eleventh Corp (O.O. Howard commanding) too far north of the city with neither flank properly anchored. A large gap between the First Corps (who had brilliantly held their own despite the loss of their excellent leader John Reynolds) and the right wing of Howard's corps was merely troubling. The left flank under command of Brigadier General Barlow with its flank "in the air" on Blocher's Knoll was unforgivable, and was hit squarely by the Confederates under General Jubal Early, who rolled up the flank and forced the entire 11th Corps through the streets of Gettysburg and up to the back-up positions on Cemetery Hill and Culp's Hill. This was the second time the 11th Corp had been humiliated in less then two months, because the same flanking enfilade attack was made on them at Chancellorsville (May 2, 1863) by Stonewall Jackson's forces.