We must stop, reflect, change ourselves and only then will we be able to make a better world. You will find The Complete Tisha B'Av Service by Rabbis Avrohom Chaim Feuer and Avie Gold helpful to understand the day and the service. It is the preparation of the Jewish people for entering and living in the Land of Israel. Moshe reviews the history of the 40 years of wandering the desert and gives rebuke so that the Jewish people will learn from their mistakes. It is always good to give reproof right before one dies.
People are more inclined to pay attention and to take it to heart.
Moshe recalls what happened at Mt. Sinai, the appointment of judges and administrators, the story of the spies, the prohibition to attack Edom and Moav, the defeat of the Kings Sichon and Og, and how the land of Gilad was given to the tribes of Reuven, Gad and half of the tribe of Menashe. Therefore, Moshe waited until he had conquered Sichon and Og, and then he rebuked the people. Had the people felt that Moshe's rebuke was insincere and that he had ulterior motives, his words would have been ineffective. A person will only accept rebuke if he feels that the rebuker has his best interests in mind. Sometimes the longest distance in the world is between the head and the heart.