There is a well known tradition (recorded in Mishnah Ta'anit 4:6) that the first Tishah b'Av was when the spies came back and ten of them gave their evil report. But there is an earlier Tisha b'Av which is less obvious.
The Zohar Chadash Ma'amar Mechirat Yosef points out (at the end of the ma'amar) that from the 17th of Tamuz until the 9th of Av is 22 days, just like there are 22 years that Joseph did not see his father Jacob (Quick math: Yosef was 17 when he was sold. Breishit 41:46 says he was 30 when he went before Par'oh. Then there were 7 good years, and then 2 bad years, hence he was 39 when Yaakov came to Mitzraim. 39 - 17 = 22).
To take the metaphor to its conclusion, the sale of Yosef by his brothers is the beginning of the troubles, the 17th of Tamuz, when the walls of Yerushalayim were breached. The beginning of the exile to Egypt, therefore, is the 9th of Av. Mechirat Yosef was the beginning of the 210 year exile.
The Midrash Tanchuma to parshat VaYigash (siman 10) notes that everything that happened to Yosef happened to Tzion (aka the Land of Israel). And indeed a long list of verses are given there to prove the point. In Kol haTor, it is noted that both Yosef and Tzion share the same gematria, 156.
The connection here is that the Sin of the (ten) Spies is a repeat of the Sin of Mechirat Yosef (by ten of his brothers). The sins are similar in nature. Yosef displayed signs of being special (both in his relationship with their father, and with his dreams), and his brothers were jealous of this. Jealousy of someone else's lot indicates a lack of faith (See the GR"A's Even Shleimah, chapter 3). Who is rich? One who is happy with his portion (Pirkei Avot 4:1). The ten spies, similar to the ten tribal heads before them, were worried about losing their leadership status upon entering Eretz Yisrael (per the Zohar). This worrying is the beginning of jealousy, as they didn't actually have anyone to be jealous of at that point, as it was all still theoretical.
This lack of faith, and in turn jealousy, caused the destruction of the House of our G-d, the desolation of our land, and the exile of our nation. As preparation for the rebuilding of the House of our G-d, we must reverse the sins which caused it in the first place. We must all settle all of our land with faith, and rebuild what has been lost.
The Beit haMikdash will be rebuilt in its time, and it is forbidden to build it too early (See the Vilna Gaon on Tikkunei Zohar). But nonetheless, much work is left to be done before that day, as everything short of the actual building is in our hands. With faith, we will learn to love each other. Each of us will then seek out his or her own mission, and no one will be jealous of the other's achievement.
ישעיה יא:יג וסרה קנאת אפרים, וצררי יהודה יכרתו: אפרים לא-יקנא את-יהודה, ויהודה לא-יצר את-אפרים