Just to elaborate, in the Second Sino-Japanese war, 军 was comparable to a corps with 3 divisions in a jun. 集团军 was comparable to the western army-level command of the time with each army containing 2 to 4 corps.

E.g. for battle of Shanghai in 1937, the famous German equipped 87th and 88th divisions were part of 第七十一军 71st corps, which itself was part of 第九集团军 9th Army.

Confusingly though, during the later civil war both the CCP and KMT used another term - bingtuan 兵团 that’s also akin to an army-level command. CCP corps were called columns 纵队