Jamieson, Fausset and Brown on Joshua 8:15-17

15-17. Joshua and all Israel made as if they were beaten before them--the pretended flight in the direction of the wilderness; that is, southeast, into the Ghor, the desert valley of the Jordan, decoyed all the inhabitants of Ai out of the city, while the people of Beth-el hastened to participate in the expected victory. It is supposed by some, from "the city," and not "cities," being spoken of, that the effective force of Beth-el had been concentrated in Ai, as the two places were closely contiguous, and Ai the larger of the two. (See Jos 12:9). It may be remarked, however, that the words, "or Beth-el," are not in the Septuagint, and are rejected by some eminent scholars, as an interpolation not found in the most ancient manuscripts.