Jamieson, Fausset and Brown on Joshua 23:12-13

Jos 23:12. By Threatenings in Case of Disobedience.

12-13. Else if ye do in any wise go back, and cleave unto the remnant of these nations--As marriage connections with the idolatrous Canaanites would present many and strong temptations to transgress it, these were strictly prohibited (Ex 34:12-16; De 7:3). With his eye, as it were, upon those prohibitions, Joshua threatens them with the certain withdrawal of the divine aid in the further expulsion of the Canaanites (a threat founded Ex 23:33; Nu 33:55; De 7:16).