Final answer:
Longstreet believed that Lee's frontal assault plan would result in heavy casualties and fail to break the Union lines.
Step-by-step explanation:
Longstreet, who was known to be unenthusiastic about the plan, did not believe that Lee's frontal assault would end in success. He was skeptical of the plan due to the strong defensive position of the Union forces and the heavy artillery and gunfire they would face during the charge.
Longstreet believed that the attack would result in heavy casualties for the Confederate troops and ultimately fail to break the Union lines.
Lieutenant General James Longstreet was not optimistic about General Robert E. Lee's plan for a frontal assault, which came to be known as Pickett's Charge. On July 3, after unsuccessful attempts to outflank the Union forces, Lee directed Longstreet to coordinate an attack on the Union center at Cemetery Ridge, despite Longstreet's reservations.
Longstreet was unenthusiastic because he likely believed that a direct assault against a fortified enemy, positioned advantageously on high ground, would lead to high casualties and might not be successful given the Union's defensive posture and their control of the terrain at Gettysburg.