וַיֵּצֵא֩ סִיחֹ֨ן לִקְרָאתֵ֜נוּ ה֧וּא וְכָל־עַמּ֛וֹ לַמִּלְחָמָ֖ה יָֽהְצָה׃
Dann trat Sihon gegen uns, ihn und sein ganzes Volk, heraus, um in Jahaz zu kämpfen.
Rashi on Deuteronomy
ויצא סיחן AND SIHON WENT FORTH — He did not send for Og to help him: this serves to teach you that they did not require one another's help, so mighty was each of them.
Siftei Chakhamim
He did not send for Og, etc. Because they were powerful.