The Roman-age Jewish historian Josephus notes that the fire that destroyed the beautiful Herodian temple in Jerusalem caught fire on exactly the same day when the Babylonians burned down the first temple. According to the Jewish calendar, this happened on the 9th of Av, which is today. The first temple was destroyed in 586 BCE, the second temple in 70 CE, 1944 year ago. That's a long time.
The reason Jews remember these events is because they are meticulously preserved in our literature, liturgy, and customs. The destruction of the first temple, a primordial political catastrophe, when the great Davidic kingdom of Judah came to an ignominious end, is enshrined in biblical literature. Prophets, historians, and poets did their utmost to prevent Jews from forgetting their past: "If I forget thee, Jerusalem, may my right hand wither," was written down in Babylon, by Jews exiled from Jerusalem, taking a vow to return and rebuild. And rebuild they did. A mere seventy years after the destruction of the temple, the altar of sacrifice was rededicated and the city of Jerusalem founded anew.
The Roman destruction of Jerusalem was more thorough. The Roman Empire lasted longer and its policies of pacification of unruly Judea were more thorough. Not only did they (eventually) plough under Jerusalem and build a pagan city (a Roman military colony) in its place. They also banished the Jews from living in or around Jerusalem, a banishment that lasted-give and take a few exceptional years-until the seventh century, when the Romans were kicked out of Syro-Palestine by the Arabs. Meanwhile, the Jews--now living anywhere from Mesopotamia (under Sassanid Persians) to North Africa, Arabia, Asia Minor, Italy and Spain--developed a piety based on law and custom that preserved the memory of Jerusalem but left its restoration to the advent of the Messiah, G-d's davidic redeemer and restorer of the great Jewish commonwealth at the end of days. Meanwhile Jerusalem was gradually converted from a Christian pilgrimage city into a typical Muslim city: a place where many nations (people of the book, nominally subjugated but effectively tolerated by Arabo-Turkish elites) cohabited, including a Jewish colony of Rabbanites and Karaites.
Traditionally Jews left redemption to G-d. The best they could do to influence the divine machinery to move from suffering an exile toward cosmic rectification was to keep the commandments. Redemption was to come when all Jews kept two Shabbatot in a row, a pretty utopian idea but not on principle beyond human initiative. In the modern age, Zionism famously determined that redemption was entirely up to human initiative. Im tirtsu, eyn zu aggadah: Wenn Ihr wollt ist es kein Märchen: If you want it it is not a dream. Thus the redemption of Jerusalem became a political program, patiently pursued. Zionism brought us the State of Israel with Jerusalem as the "eternally undivided capital" of Israel. Why then are we still fasting on the Ninth of Av?
Religious people fast because it is commanded. And they fast because the temple is still in ruins, so to speak. In any case, it has not been rebuilt. There may be other reasons to fast in this day and age. Maybe even perfectly secular ones. For example, fasting to acquire the wisdom necessary to preserve the modern Jewish commonwealth. Ancient Jewish politics brought down the ancient Jewish polity. I am fasting in commemoration of ancient folly and in the hope that we can learn from history.