The Talmud (Gittin 55b-56a) states that there were two men in Jerusalem, Kamtza and Bar Kamtza. When a prestigious man made a great feast and invited all the leaders of Jerusalem, among them was his good friend Kamtza. However, the messenger made a mistake and inadvertently invited Bar Kamtza, who happened to be the host's enemy. When the host came and saw Bar Kamtza sitting there, he asked him why he was present. Bar Kamtza replied that he was invited, and since he was already there, he wished to remain so as not to be embarrassed. He even offered to pay for anything he ate or drank. The host refused. Then Bar Kamtza offered to pay for half the feast. The host refused again. Then Bar Kamtza offered to pay for the entire feast. And yet again, the host refused and ordered Bar Kamtza to be physically thrown out of the banquet hall.
Bar Kamtza thought to himself that since there were many Torah scholars present who remained quiet, they must have agreed with what had transpired. In disgust, he sent a message to the king that the Jews were rebelling against him. To show that he was telling the truth, Bar Kamtza told the king to send a sacrifice to the Beit Hamikdash (which was allowed) and see if they sacrifice it. So he sent a sacrifice through Bar Kamtza, but along the way Bar Kamtza made a small cut on the animal (which would disqualify it). When Bar Kamtza brought the sacrifice to the Sages, they said that even though the animal was blemished, it should nevertheless be sacrificed for the sake of peace with the Roman empire.
However, one Torah scholar named Rabbi Zechariah ben Avkulas dissented by arguing that people would think it was acceptable to offer a blemished sacrifice. Therefore, the Sages ruled that Bar Kamtza would have to be killed so that he could not report back to the government that the sacrifice was refused. But Rabbi Zechariah again dissented by arguing that people would think that anyone making a blemish on a sacrifice would be killed. After caving in to these misguided priorities, Roman attacks ensued. The rest is history. Many years later, Rabbi Yochanan said that because of the zealousness of Rabbi Zechariah ben Avkulas, the Beit Hamikdash was destroyed and we were exiled from our land.
In summary, what all started with one instance of indecent treatment toward a fellow Jew led to that person denouncing his people to the Roman emperor, which then led to poor rabbinic decisions (i.e. favoring technical legal details above common sense), which finally led to the destruction and exile. In order to undo this, we have to correct the problem at its source. The only way we can possibly merit the Final Redemption is by redeeming ourselves first. In other words, just as Jewish history reached its lowest point when hatred and dissension became ubiquitous, we will witness our greatest days when good behavior and common sense become ubiquitous.
Our Sages taught us that every generation in which the Beit Hamikdash is not rebuilt, it is as if it was destroyed. However, they are actually giving us a bit of motivation. Every time you have the opportunity to act indecently toward a fellow Jew and don't, you're literally contributing to the rebuilding of the Beit Hamikdash. God wants to redeem the world, but He needs us to do one thing, and it really isn't all that much to ask: become better people. Simply heed the words of Micah 6:8 - do what's right, be kind, and remain humble. Really, that's it. Let's get back to basics and correct this problem once and for all. When we succeed, Tisha B'av will be transformed from a day of commemorating our greatest tragedies to a day of celebrating our greatest triumphs.