I feel I have to challenge the opening sentence: 'The alternate history genre grew, ultimately, out of portal fantasy stories in science fiction magazines.' If we look at the history of alternate history fiction, we see in fact it grew out of political tracts. Work by Livy, Gibbon, Delisle de Sales, Geoffroy-Chateau, Holford, Pidgin, Munro, then blossoming in the third and fourth decades of the 20th Century with Burdekin, Sprague de Camp (he features Fascist Italy as a starting point), Jameson, Morton, etc., etc. all of these were either political books or political history books or developed scenarios based on such takes on political occurrences. These were the foundations of alternate history writing down the centuries and into the mid-20th Century.
