The Jewish tradition teaches that the liturgical reading of the fifth of the books of the Torah or Pentateuch, Devarim, should begin on the Shabbat before the fast on the 9th of Av (Shuljan Aruj, Oraj Jayim 428: 4). The first parashah of the book recounts how the Children of Israel refused to fight for their land after they had received the report of the spies they had sent into it. According to Talmudic sources, this episode also happened to occur on the disastrous ninth day of the month of Av, the date remembered as when the First and Second Temples were destroyed. (Mishnah Taanit 4: 6).
The Book of Devarim, Deuteronomy, codifies laws about the organization of the people in their land: the judicial and policing power, the rights and obligations of monarchs, ethics and the military code, as well as other laws from the civil, commercial, and criminal fields that regulate the daily life of the nation and its citizens.
Of particular importance is the fact that Deuteronomy restricts sacrifices to God to the Temple in Jerusalem. The ritual is almost totally centralized in one place; sacrifices were prohibited to be offered outside it. Ultimately, this was the factor that allowed Jewish spiritual development at the time of the second Temple (Y. Kaufman, Toledot ha-Emunah ha- Israelit, Volume 1-3, page 201 and following)
The destruction of Jerusalem and its Second Temple in the year 70 by the Roman legions led by Titus meant the loss of everything connected to Jewish sovereignty over the land of Israel and its life as a sovereign people.
The text of Devarim is Moses' last great speech to all the people. These are the words of God interpreted by the great prophet (Meguila 31, b), which marks the beginning of the oral interpretative tradition of the Torah text (Megale Tsefunot by Rabi Elijah ben Salomon Abraham ha-Kohen, Devarim). It sees the holiness of the Torah, in the book of Deuteronomy, as the result of its being a joint composition of God with human being. This teaches that
God's Scripture requires human interpretation for its application. The Torah and its study is one of the factors that have maintained the existence of the Jewish people while it was dispersed in the Diaspora for two millennia.
Shabbat shalom!