Dempsey points out, as have others, that Williams deserved promotion to the rank of major general, given his good reputation and record in war. But the fact Williams was not a West Pointer worked against him, and it appears he did not cultivate the political support or seek the publicity he needed. Thus, he held his major general’s post with only brevet, or honorary rank, while proving himself a popular and respected general in the Union army’s Second, Twelfth and Twentieth Corps, with his soldiers giving him the nickname “Pap” (though pronouncing this “Pop”.)
"Michigan’s Civil War Citizen-General: Alpheus S. Williams,"
Civil War Book Review: Vol. 22
Available at: https://digitalcommons.lsu.edu/cwbr/vol22/iss1/17