The Jewish calendar is set so that פרשת דברים is always read on the Shabbat before תשעה באב. This Shabbat is also called שבת חזון because of the Haftarah that begins with the words, חזון ישעיהו.
What the Parsha and Haftarah have in common, is that they both are filled with תוכחה, rebuke.
In פרשת דברים, Moshe Rabbeinu reviews the numerous places the Jewish people traveled during their forty years in the desert. He reminds the nation of the various places where the Jewish people rebelled. Moshe was one of several leaders who used their impending death to rebuke the people.
The Prophet Isaiah similarly rebukes the nation by telling them that even their animals show more obedience to their masters than the Jewish people were to Hashem.
It is very difficult to know how to give תוכחה. But it might even be more difficult to be able to receive תוכחה. The Tanna, רבי טרפון, once said that he was amazed if anyone ever accepted rebuke.
The reason why there is a Tisha B’Av, is because we did not pay attention to all of the warnings, and we rebelled against Hashem. In order for the Redemption to come, we must learn the lessons from our past, and sincerely return to Hashem and the Torah. Shabbat Shalom