The gemara (R”H 18b) cites Rav Papa as explaining that the obligation of fasting depends on the state of peace/war the Jewish people find themselves in: 1) if there is peace (Rashi: Jewish control over their own lives), then there is no need to fast; 2) if there is oppression, the fasts are obligatory; 3) if there is neither complete peace nor a state of oppression, the obligation is dependent on the people’s choice. The poskim write that generations ago the practice of fasting was adopted by the Jewish people, so one can no longer “opt out”. The gemara challenges Rav Papa: the Mishna (R”H 18a) tells us that messengers from Bais Din would be sent out to inform the distant communities when Rosh Chodesh Av was declared so that they would be aware of the proper day Tisha b’Av falls on. How could the Misha tell us a blanket rule that the messengers of Bais Din would always go out to inform the people of Rosh Chodesh Av in preparation for fasting when if the people chose or there was a state of peace there would be no obligation to fast?! The gemara answers that Tisha b’Av is different because it is a day of many compounded tragedies (huchpelu bo tzaros).
There are two different ways one can understand the gemara's answer (see Aruch laNer):
1) Since 9 Av is a day of multiple tragedies, Rav Papa’s halacha that says fasting is a matter of choice does not apply to 9 Av – fasting on 9 Av is always obligatory.
2) Since 9 Av is a day of multiple tragedies, the messengers of Bais Din went out expecting the people to adopt the fast (or at least be aware of the day), but Rav Papa’s statement remains true even with respect to 9 Av.
The Rambam (Hil Ta’anis ch 5) seems to group 9 Av along with the other fast days with no distinction (yet also interestingly, the Rambam does not quote Rav Papa’s halacha in that chapter). Yet, it would seem that Tosfos disagrees. The gemara (Megillah 5b) tells us that Rebbi either wished to uproot the fast of 9 Av completely or to uproot it if it fell on Shabbos (meaning one would not have to fast on Sunday). Tosfos asks how Rebbi could do this when an enactment of a previous Bais Din cannot be overturned by a later one. If Rav Papa’s halacha applied to 9 Av, then Tosfos’ question is moot - the entire takana of fasting is subject to the people’s will! It seems that according to Tosfos the fast of 9 Av is obligatory independent of the people’s choice.