Is General Longstreet wrong to obey Lee's orders, even though he disagrees? Would it have been better for Longsteet to try to convince other generals to change Lee's mind? And why doesn't Longstreet try to take command from Lee?
Both of the previous posts are dead on concerning Longstreet's decision to obey Lee's order for Pickett's attack. A refusal to obey would have constituted gross insubordination, and Lee would likely have had to arrest or relieve his "Old War Horse." Longstreet admired Lee too much to have considered trying to undermine him by attempting to convince his division and/or brigade commanders to defy the order. Longstreet was a military man (unlike many of the non-West Point commanders of lower rank), and I'm sure he would have resigned rather than fuel a possible mutiny. He probably would have had little support anyway: Robert E. Lee had already achieved legendary status among his troops (and, for that matter, throughout the continent and most of the world), and the Army of Northern Virginia had been established as the foremost fighting outfit on the planet. Their confidence (or overconfidence) would prove to be misplaced after Gettysburg, but prior to the Pickett's Charge, few of Lee's soldiers believed they could be beaten. George Pickett was ecstatic at the chance to lead the assault, and he apparently did not envision the futility of the attack as his friend Longstreet did. After the war, Pickett was often quoted speaking derogatorily of Lee's decision, but he was eager for his chance at glory and everlasting fame on the day of the attack.
Although Lee's decision on the frontal assault was probably doomed from the start, had Longstreet taken more positive steps to assure a chance of success, the outcome may have been different. The brigades attached from Gen. A. P. Hill's corps had already seen extensive fighting, and they were not the best choices for aiding in the attack. Several of Hill's other brigades had seen little action and were larger and fresher; they would have been better choices to join the charge. Pickett's choice as leader of the attack also has to be questioned, since his division was one of the least experienced groups in Lee's army. Pickett stayed well in the rear of the attack and has been criticized for his lack of front-line supervision (though his position in the rear was not militarily improper and leading from the front would probably have only caused him to be another casualty). Longstreet, always a slow and deliberate planner, apparently delayed the attack in the hope that Lee would change his mind. Lee would later take the entirety of the blame after the failed charge--"It's all my fault," he told his men--but history would also place everlasting blame on both Longstreet and Pickett as well.
I think that the previous post is dead on accurate. In a military configuration, the chain of command is sacred. It is absolute. It has to be respected. Personal opinions are a distant second to following orders. In deference to this, Longstreet could not have sought to pursue insurrectionist ends against this chain of authority. In terms of the secondary point of trying take command from Lee, there are some inherent problems here. In a military situation and in a wartime situation, it is viewed in horrifically bad character to attempt to engage in a power struggle. The structure is there and one must operate in it. The orders given were not illegal, and they were in the bounds of war. If individuals in a military unit begin to question orders that are given, there is complete disarray and victory will be lost, with more lives becoming casualties. In this light, Longstreet had no choice but to respect Lee's orders, personal feelings aside. Perhaps, he could have confided his feelings to Lee in private, with the permission to speak freely, but acting upon these is something that cannot be tolerated in a military and warfighting situation.
I assume that you are talking about the time when Longstreet disagreed with Lee as to whether Pickett's division should mount a frontal assault on the Union positions. In my opinion, Longstreet did what he should have and the idea of trying to convince other generals to change Lee's mind is a bad one.
In a military organization, as long as an order is legal, it must be obeyed. The people in command are presumably there on merit and therefore their subordinates must not second guess them on tactical issues. Longstreet argued with Lee, but once ordered to have Pickett charge, he had no choice. If he had done anything else, it would have been, more or less, mutiny. If he really felt what Lee was doing was a terrible idea, he should have told Lee he could not do it and ask to be relieved of command.