Final answer:
The attack on Gettysburg known to be a big mistake is Pickett's Charge, defined by a high casualty rate and being the "High Watermark of the Confederacy".
Step-by-step explanation:
The attack on Gettysburg that is widely considered to have been a big mistake is A) Pickett's Charge. This was a bold infantry assault led by Major General George Pickett on the third day of the Battle of Gettysburg, July 3, 1863. After unsuccessful attempts to outflank the Union army, Confederate General Robert E. Lee ordered an attack directly up the middle, targeting the Union center on Cemetery Ridge. Despite the magnitude and audacity of this classic military maneuver, it was not surprising to Union General Meade, except for the timing and the leader of the assault.
The Confederate soldiers marched across open fields toward entrenched Union forces, facing devastating artillery and rifle fire. The high casualty rate of Pickett's Charge contributed to it being labeled the "High Watermark of the Confederacy", with approximately half of the 12,500 Confederate men being wounded or killed. This charge was indeed the climax of the Battle of Gettysburg, and was instrumental in turning the tide of the war in favor of the Union.