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 of Parshas Va'eschanan, which is read on Tisha B'Av itself, appears immediately after this week's reading. Why was the tochachah of Va'eschanan, or some other admonition, not designated for the period leading up to Tisha B'Av?
I think that the answer lies in the unique and exact content and context of Moshe's words in Parshas Devarim. The overall structure of this week's parshah delineates the circumstances in which Bnei Yisroel find themselves at the end of the Midbar experience. The old generation had basically died out as a result of the Sin of the Meraglim. If not for that event, the Jews would have already been in Eretz Yisroel for 38 years, with the previous dor still in control. Due to the Sin, entry to the land was 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 Sin of the Meraglim, so as to provide a context to the people. He lets them know where the Jews were headed prior to the Sin, and he explains the total shift of course and eternal ramifications which the Sin precipitated, thus placing the new dor at the point of entry to the land 38 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 Sin decreed. Jewish destiny was forever changed on the Ninth of Av in those days. This is precisely why we read Moshe's rebuke of 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 endures 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 dor of Devarim.
(As was explained in the d'var Torah on Shabbos/Chol Ha-moed, the Shabbos of each special period in the calendar is devoted to the underlying philosophical theme of the period. This is why Shabbos "Chazone", which occurs during the pre-Tisha B'Av period of halachic mourning, is devoted to the kriah from Parshas Devarim. On Tisha B'Av itself, however, we read texts which relate to the historical side 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 in our destiny and the redemption.