Final answer:
General Lee and Confederate strategists planned a Northern invasion to weaken the Union's power and force negotiation. Lieutenant General Longstreet believed this plan would force the Union to attack.
Step-by-step explanation:
According to the given information, General Lee and Confederate strategists planned an invasion of the North in the hopes of weakening the Union's power and forcing them to negotiate peace. Lee believed that a significant Confederate victory in the North would also convince Britain and France to support the Confederacy.
This plan would force the Union to send troops from the Vicksburg campaign to the East, which would weaken their control over the Mississippi.
On the other hand, Lieutenant General Longstreet, who was not enthusiastic about the plan, believed that the Union forces would be forced to attack due to the Confederate invasion.
He thought that the aggressive action of pushing north would reduce the Union's resolve to fight. However, the delay in the attack and organizational issues resulted in unnecessary casualties for the Confederates.