Why does it say "Asah Hashem Asher Zamam"?
Rav Sadya Gaon: Like He decreed, so He did. Ri Kara - in many places, He retracted from His decree. Here, He did His intent.
Palgei Mayim: He fulfilled all the curses in the Tochachah, and even those that were not written, they were merely intended - "Gam Kol Choli v'Chol Makah Asher Lo Chasuv b'Sefer
What is the meaning of "Bitza Emraso"?
Rashi: He finished His decree. This is like "Yater Yado vi'Yvatze'eni" (Iyov 6:9).
R. Avigdor citing Eichah Rabah 2:20 #1: Bitza is an expression of Bitzu'a (compromise).
R. Avigdor citing Eichah Rabah 2:20 #2: We read this like Imraso, an expression of Imra (the fringe of a garment) - it is as if he tore his royal garment. A Mashal for this is a Chacham who never saw Aveilus. His son died. He asked his Talmidim, show me how people mourn. They said, he tears his garments 1 and sits silently, alone. He has an angry face, wears black garments, and sits and cries the entire day and night. He said, so I will do.
Palgei Mayim: He divided His words. It says in the Tochachah "Ad Hishamedcha" (Devarim 28:20). He nullified this, and poured out His anger via destroying the Beis ha'Mikdash without mercy, rejoicing the enemy over it and elevated the horn of your enemies.
R. Avigdor: Some ruled not to wear a Talis on Tish'ah b'Av. This is a mistake. An Avel is obligated in all the Mitzvos, including Tzitzis, even on the first day [after the burial; before burial, he is an Onen, and he is exempt - PF]. Even if the Midrash says that he tore his garment, he did not remove it; he wore it torn. Also, if so they should tear their garments on Tish'ah b'Av! Rather, one is obligated in Talis on Tish'ah b'Av, and even in Tefilin (even though an Avel is exempt from Tefilin on the first day - PF).
When was this commanded earlier?
Rashi: "V'Yasafti Leyaserah Eschem Sheva
What is the meaning of "va'Yasamach"?
Rav Sadya Gaon: He made [the enemy] rejoice.