וַיֵּצְאוּ֙ בְּנֵ֣י עַמּ֔וֹן וַיַּֽעַרְכ֥וּ מִלְחָמָ֖ה פֶּ֣תַח הָעִ֑יר וְהַמְּלָכִ֣ים אֲשֶׁר־בָּ֔אוּ לְבַדָּ֖ם בַּשָּׂדֶֽה׃
Und die Kinder Ammon kamen heraus und stellten die Schlacht am Tor der Stadt auf; und die Könige, die gekommen waren, waren allein auf dem Feld.
Rashi on I Chronicles
were by themselves in the field They divided their armies into two battle arrays: the children of Ammon were in one array at the entrance of the city against all Israel, and the kings who were with them in the second array were by themselves in the field behind Israel.