Albert Castel, "Williams, Alpheus Starkey," American National Biography Online, February 2000, http://www.anb.org/articles/05/05-00839.html.
Williams's greatest and most crucial performance as a combat leader took place on 2-3 July 1863 at Gettysburg, where he again was acting commander of the XII Corps. On the afternoon of 2 July, having been ordered to bolster the Union left against a threatened Confederate breakthrough, he pulled his troops from their works on the Union right and successfully accomplished his mission. Then, on returning that night to his former sector, he discovered that a strong enemy force had occupied his empty trenches, putting itself in position to seize Culp's Hill, the loss of which would have led to Union retreat and probable defeat. Wisely refraining from a night attack, Williams waited until daylight, then mounted an assault that Edwin B. Coddington described as "well-conceived" and "efficiently executed," with the result that the Confederates were driven back and the danger to Culp's Hill eliminated. Few northern generals contributed more to the Union victory at Gettysburg than Williams.