Parshas Devarim is always read on the Shabbos right before Tisha B'Av. The Biur Halacha (528:4) explains that this is purposefully done in order that Moshe Rabbeinu's admonition to B'nei Yisroel coincide with the Tisha B'Av period.

One may ask why it is necessary for the specific admonition in Parshas Devarim to be read in conjunction with the mourning period which precedes Tisha B'Av. After all, the tochachah (admonition) of Parshas Va'eschanan, which is read on Tisha B'Av itself, appears immediately after this week's reading. Why was the tochachah of Parshas Va'eschanan, or some other admonition, not designated for the Shabbos prior to Tisha B'Av?

It would seem that the answer lies in the unique content and context of Moshe Rabbeinu's words in Parshas Devarim. The overall structure of this week's parshah delineates the circumstances in which B'nei Yisroel find themselves at the end of the Midbar (Desert) experience. The old generation had basically died out as a result of the Chet ha-Meraglim (Sin of the Spies). If not for that event, the Jewish People would have already been in Eretz Yisroel for almost 40 years, with the previous generation still in control. Additionally, as explained by Rashi (on Devarim 1:8), if not for the Chet Ha-Meraglim, Eretz Yisroel would have been gifted to B'nei Yisroel without the need for warfare. But now, as a result of the Chet, entry to the Land was extremely delayed, the old generation was mostly extinct (the Levi'im and women survived), and the entirety of Jewish destiny was redefined. Moshe therefore addresses the present generation from this perspective, devoting most of the parshah to the Chet Ha-Meraglim, so as to provide a context for the people. He lets them know where B'nei Yisroel were headed prior to the Chet, and he explains the major shift of course and eternal ramifications which the Chet precipitated, thus placing the new generation at the point of entry to the Land almost 40 years after conquest should have already occurred.

The masses cried in vain as a result of the Spies' false report on the night of Tisha B'Av, and on Tisha B'Av was punishment for the Chet Ha-Meraglim decreed. At that point, on the Ninth of Av, Jewish destiny was forever changed.

This is precisely why we read Moshe's admonition in Parshas Devarim on the Shabbos before Tisha B'Av. We are again told that the present situation in which we find ourselves - a Jewish destiny of golus and suffering - was caused by grievous transgressions long ago, just like the generation of Devarim was advised by Moshe that its parents' actions irrevocably altered its destiny. We are instructed that the thousands of years of suffering must be traced back to Tisha B'Av, for that horrific day marked the beginning of a chain of exile and punishment which form the spine of our history and endure to this very day. We did not live through the churban of Yerushalayim, but its impact defines our existence, just as the impact of the Sin of the Spies defined the state of being of the generation of Devarim.

(As was explained in the d'var Torah about Shabbos Chol Ha-Moed, the Shabbos of each special period in the calendar is devoted to the underlying hashkafic theme of that period. This is why Shabbos Chazon, which occurs during the pre-Tisha B'Av period of mourning, features the tochachah of Parshas Devarim. On Tisha B'Av itself, however, we read texts which relate to the historical aspect of the day - as is the case on all Moadim - and we thus recite Eichah and the tochachah from Parshas Va'eschanan.)

May we soon merit to find ourselves in the fulfilled context of "Nachamu, nachanu ami", as we patiently await the final realignment of our destiny and the long-anticipated, wondrous Redemption.