Wikipedia has a decent overview over early conquests of the Islamic empire. The expansion seems to have been one of the most rapid in history. I'm not aware of any Muslim scholar who opposed the expansionism that was pursued by the caliphate, neither that of the Rashidun, the Umayyads, or later caliphs.
However, as far as I can tell, the dominant trend on islam.se seems to be to claim or suggest that warfare in general is a purely defensive construct in Islam, see e.g. here or here or here. If that is correct, then the ulama likely opposed Islamic expansionism on doctrinal grounds, and no scholars would have condemned them as kuffar for doing so.
Question: if any, which ulama opposed Islamic expansionism, what were their arguments for doing so, and what was the reception of their ideas in among the other ulama?