In General A.A. Humphrey's, "Virginia Campaigns," page 40, we read:
"At seven a.m., General Hancock sent a staff officer to General Gibbon, informing him of the success of his right wing, and directing him to attack the enemy's right with Barlow's division. This order was only partially obeyed. Had Barlow's division advanced as directed, he (General Hancock) felt confident that the enemy's force would have been defeated. The cause of his failure was probably owing to the expected approach of Longstreet on his (Barlow's) left."
Again:
"At 8:30 a.m., Hancock began an attack with Birney's wing and Gibbon's division of the left wing."
General Grant, in his memoirs, (pp. 196-197):
"Hancock was ready to advance, but learning that Longstreet was threatening his left flank, sent a division of infantry, commanded by General Barlow, to cover the approaches by which Longstreet was expected."
General Sheridan, (memoirs, vol. I, pp. 362-363):
"On the sixth, General Meade became alarmed about his left flank and sent a dispatch, saying: 'Hancock has been heavily pressed and his left turned. You had better draw in your cavalry to protect the trains.'"
And again:
"On the morning of the sixth, Custer's and Devin's brigades had been severely engaged before I received the above note. They had been most successful in repulsing the enemy's attacks, and I felt that the line could be held. But the despatch from General Hancock was alarming, so I drew all the cavalry close in around Chancellorsville."