On April 12, Lieutenant Pierre G. T. Beauregard, of the United States Army Corps of Engineers, determined that possession of Atalaya Hill would enable the Mexican position to be turned. The "Personal Memoirs of Ulysses S. Grant" observe that, in order to determine whether a flanking movement was possible, "reconnaissances were sent out to find, or to make, a road by which the rear of the enemy's works might be reached without a front attack." These reconnaissances were made under the supervision of Captain Robert E. Lee and other officers, "all of whom attained rank and fame." Grant continues that it was the roadways constructed by the engineers which achieved victory: