If you will research this a bit, I think that you will see that Custer was indeed given orders by Gen. Terry, and failed to follow them.Had he listened and done what Gen. Terry told him to do, Custer may have not had his last stand on that day.
"He (Gen. Terry), gave Lt. Col. Custer EXPLICIT orders to lead the 7th Cavalry up the nearby Rosebud River to arrive no sooner than 26 June to allow Gibbon's troops, slowed by the Gatling Gun Division, time to take up their positions.
Custer paid little heed to Gen. Terry's instructions and soon after departing up the Rosebud, headed directly for the valley of the Little Big Horn making forced marches late into the night and starting again before dawn.With his 7th Cavalry troopers trail-weary and his horses exhausted, Custer reached the valley early in the afternoon of 25 June and made plans to attack the Indians immediately.Custer's Indian scouts warned him that Sitting Bull's camp was too large for him to take on with his small troop but Custer thought that his 7th Cavalry could whip any Indian war party.Custer divided his troops into 3 separate commands placing 125 men under Capt. Benteen with instructions to move towards the foothills and fight any Indians that he found.A 2nd battalion under Maj. Marcus Reno was sent to engage the Indians in the village across the Little Big Horn with Custer to follow up with his battalion and provide whatever support was needed."
Information can also be found at this site under the heading, "Enter George Armstrong Custer":
"While Custer no doubt had experience, there were those who were superiors and subordinates who feared that Custer lacked the judgment needed to face a man like Sitting Bull on the battlefield.
Custer had experienced conflict with both his commanders in the Dakota Dept., and within his regiment.
When Gen Terry decided to send his cavalry to "scout the trail" reported by Reno, Custer was given command of the expedition.
Terry's orders to Custer showed an unusual combination of anxiety and tolerance.He seems to have feared that Custer would be impetious, but he resisted issuing an order that might wound the high spirited commander of the 7th Cavalry.Terry warned Custer to keep watch well out toward his left as he rode westward from the Rosebud, in order to prevent the Souix from moving southeastward between the column and the Big Horn Mountains.................but, he named the 26th of June as the day on which he and Gibbon would reach the valley of the Little Big Horn, and it was his hope and expectation that Custer would come up from the east about the same time, and between them they would be able to soundly whip the assembled Indians.
Custer let him down in an unexpected way.He got there a day ahead of time, and had ridden night and day to do it.Men and horses were exhausted when the 7th Cavalry rode into sight of the Indian Village on the Little Big Horn that cloudless Sunday morning on the 25th.When Terry came up on the 26th, it was all over for Custer and his regiment.
Custer started on the trail with the 7th Cavalry, and nothing else.A battalion of the 2nd was with Gibbon's column; but luckily for the 2nd, Custer wanted none of them.Two field guns were with Terry, but Custer wanted his own people.He rode 60 miles in 24 hours.He pushed ahead with focus and without hesitation.He created an impression that he wanted to have one dramatic battle with the Indians, in which he and the 7th would be the only participants, and hence the heros.The idea that he could be defeated apparently never crossed his mind.Custer sought glory, but in the end, found only infamy.
Crook had over 2,000 men only 30 miles to Custer's left.If Custer had been scouting as instructed, he would have run into Crook's outposts, and Crook could have reinforced him.Custer wanted nothing of the sort, and was savoring the chance to have all the glory to himself.At daybreak his scouts had come across two or three warriors killed in the fight of the 17th, and they sent back word that the valley of the Little Horn was in sight ahead, and there were "signs" of the Indian camp.
Custer then decided to divide his column.He kept 5 companies, commanded by close friends, with himself.He left Capt. McDougal with some troops to guard the rear.He divided the remaining companies between Benteen and Reno.Benteen was sent 2 miles to the left, and Reno remained between Benteen and Custer.This formed 3 small columns of 7th Cavalry, which moved quickly westward over the divide."
As you can see, all historians note that Custer failed to follow Terry's orders/instructions.