I noticed that with respect to describing the fall of Betar, which took place on 9 Av, the Rambam (Ta'anis 5:3) writes:
ונלכדה עיר גדולה ובית תור היה שמה והיו בה אלפים ורבבות מישראל, והיה להם מלך גדול ודימו כל ישראל וגדולי החכמים שהוא המלך המשיח, ונפל ביד גויים ונהרגו כולם, והייתה צרה גדולה כמו חורבן המקדש
Why does the Rambam needs to add that the calamity was so great that it was like the churban? Isn't that derash, not halacha?
I haven't looked around to check if anyone says this, but it sounds like the Rambam holds that the mourning of 9 Av was instituted specifically for churban-related events. The fall fall of Betar, as great a calamity as it was, is thrown into the mix only because it was an event that in the eyes of Chazal could be equated with the churban itself. Would the Rambam agree with Rashi (Divrei haYamim II 35:25) that ALL tragic events should be commemorated in the kinos of 9 Av?