The real key of the battle-field now was the eminence at Hazel Grove.

So long as we held it the enemy could not advance without presenting his right flank to our batteries.

If he obtained possession of it he could plant guns which would enfilade Slocum's line and fire directly into our forces below.

Birney's division at this time was posted in advance of Best's guns on the left, Berry was on the right, with Williams' division of the Twelfth Corps behind Birney, and Whipple's division in rear of Berry.
The position of Hazel Grove commanded Chancellorsville, where all the roads met, and which it was vital to Hooker to hold.

For if he lost that, he could not advance in any direction, and only his line of retreat to the Ford would remain open to him.