וַיֹּ֙סֶף֙ אַף־יְהוָ֔ה לַחֲר֖וֹת בְּיִשְׂרָאֵ֑ל וַיָּ֨סֶת אֶת־דָּוִ֤ד בָּהֶם֙ לֵאמֹ֔ר לֵ֛ךְ מְנֵ֥ה אֶת־יִשְׂרָאֵ֖ל וְאֶת־יְהוּדָֽה׃
און דער צאָרן פֿון גאָט האָט װידער געגרימט אױף ישׂראל, און ער האָט אָנגערעדט דָוִדן אַקעגן זײ, אַזױ צו זאָגן: גײ צײל איבער ישׂראל און יהוּדה.
Rashi
Adonoy's anger was again kindled against Yisroel. I do not know what it was [they did].
Ralbag
...The sin was in [the fact] that David, the anointed one of the God of Jacob and the sweet singer of Israel, placed his trust in the flesh of his forearm - upon his [having] many people. And it was not fitting that he should place his faith 'in many people [that] is the glory of the king,' but rather only in God. For there is nothing that prevents Him from saving through the many or the few...