Thomas Aquinas Commentary Lamentations 2:17

"Jehovah hath done that which he purposed; he hath fulfilled his word that he commanded in the days of old; He hath thrown down, and hath not pitied: And he hath caused the enemy to rejoice over thee; he hath exalted the horn of thine adversaries." — Lamentations 2:17 (ASV)

Here he presents the approval of the judge.

1. His consistency is shown in doing what He proposed: the Lord has done that which He purposed. This is the counsel, that I have purposed (Isaiah 14:26).
2. The veracity of His speech is shown: He has fulfilled His word, for He decreed the destruction of the people in the days of old (Deuteronomy 28:20), as declared by previous prophets. Has He said then, and will He not do? Has He spoken, and will He not fulfill? (Numbers 23:19).
3. The justice of His action is shown: He has destroyed Judea and has not spared, thereby not abandoning the rigor of justice. He has destroyed the horn, that is, their power. You have set up the right hand of them that oppress him (Psalms 88:43).