Jewish history records that the 17th of Tammuz was the day on which the walls of Jerusalem were breached and this led to the eventual destruction of the temple. That seems rather unpersuasive. How often did bad things happen to the Jews? (rhetorical question -- clearly, the answer is "often"). Do we fast or commemorate each day? No. If we did, there wouldn't be a day on the calendar on which we could eat and demand for kosher corn dogs would plummet. The rabbis were not interested in shorting corn dog futures so they didn't institute myriad fasts. So why the 17th? Do we celebrate every incremental victory? No. So why fast for this one particular incremental defeat? Yes, the walls were breached and this is bad, but the destruction was in 3 weeks (ignoring the Jerusalem Talmud's view that the 1st temple's walls were breached on the ninth). We fast on Yom Kippur at the END of the days of repentance, not on each day along the way. We celebrate on Purim because that was the day of the victory, even though there were other happy moments along the way.
The Mishna in tractate Ta'anit actually records that there are 5 things that happened on that day which cause the day to be a fast. I'll list them in the order in which they are listed in the mishna:
Now, it would be easy to say "each of these deserves some sort of national mourning, so, hey, let's fast." But it isn't so simple. Take a look at 4 of the events:1. The Luchos were broken;
2. The bringing of the Tamid was annulled;
3. The walls of Jerusalem were breached;
4. Apostemus burned a Sefer Torah;
5. An idol was placed in the Beis ha'Mikdash.
1. the tablets upon which the first set of the 10 commandments was inscribed were destroyed.
No sweat. we got another set.
2. The bringing was annulled.
OK, we have prayer which takes the place of sacrifice.
3. Some guy burned a torah scroll. Sadly, it has happened many times. One version says that he burned all the torah scrolls he could find to remove torah study from the Jewish people.
Joke's on him. We're still here and still studying the torah.
4. An idol was placed in the temple (depending on the way you understand it, it refers to an action by the same guy or by king Menashe).
Either way, we have persevered.
As a matter of fact, each of the other 4 events is sort of temporary -- a step towards sadness which, by itself, is surmountable. So, again, why fast? If all the reasons are temporary obstacles which we moved past, why fast?
I think that the answer is in the dual nature of fasting in Judaism.
Fasting is often about commemorating sad events. The fast on the 9th of Av is all about marking a day of the destruction of the temples in Jerusalem. In that case, the fast on the 17th makes no sense. Sure it is sad, but so is every day! We're Jews. We suffer perennially.
But there is another dimension to fasting. Fasting is a community's way of looking at itself and its behavior and deciding to do better. And I think that this day is about the potential for success in improving. This isn't a fast of mourning but of waking up and fixing a problem.
The walls were breached, true. But we still had time to repent and ask god to forgive us. We could have done the proper actions of repentance and (like in the case of the two tablets) been forgiven and given another chance. The fact is, we didn't. We cry now because of a missed opportunity. Why do we mark a singular moment during a long process? Because that was a moment at which we could have changed things. We could have turned around and avoided the next step in the process but we chose not to. And each year, we are being reminded of this opportunity. Each year, on the 17th of Tammuz, we are given the chance to turn around and fix things. The community is shown a date and told "this is just one point on a progression -- be introspective and fix things so you can avoid the next date, the ninth of Av!"
I am fasting both as a commemoration of a missed opportunity thousands of years ago, and each year since then. I am fasting to help refocus myself so that next year I can say "this sad day turned into a moment when I turned things around. My fasting got me to rethink my position so I could help bring about the redemption, not extend the exile." I'm fasting to mark the past and help improve the future.